Method and apparatus for cooperative wireless communications

ABSTRACT

A method and apparatus for cooperation in wireless communications. Cooperation is considered among a number of network elements, including at least one wireless transmit-receive unit, at least one relay station, and at least one base station.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.61/018,571 filed on Jan. 2, 2008, 61/018,630 filed on Jan. 2, 2008,61/043,295 filed on Apr. 8, 2008, 61/046,768 filed on Apr. 21, 2008, and61/098,678 filed on Sep. 19, 2008, which are incorporated by referenceas if fully set forth.

TECHNOLOGY FIELD

The present disclosure is related to wireless communications.

BACKGROUND

Cooperative communication enables wireless transmit/receive units(WTRUs) to assist each other in transmission of information to theirdesired destination. Such an approach enables mitigation of severalissues facing modern wireless communication systems without the costassociated with extensive wired infrastructure. Using cooperation, it isalso possible to exploit the spatial diversity associated withtraditional multiple-input multiple-output (MIMO) techniques withoutrequiring each node to have multiple antennas. Finally, regenerativerelaying, a basic cooperative technique, may reduce the effects of pathloss and shadowing on coverage and throughput.

A challenge in incorporating cooperation into modern wireless systems isthe need to evolve the system architecture to enable cooperation.Effective cooperative techniques, especially in wireless systems,usually involve advanced algorithms at the lower layers of thecommunications stack, for example layer 1 (physical layer, or PHY) andlayer 2/3 (medium access control (MAC), radio link control (RLC), orlogical link control (LLC)—depending on the system). However, suchalgorithms require advanced techniques in receiver design,error-correction lisecode design, automatic repeat request (ARQ) andhybrid automatic repeat request (HARQ) processes and scheduling inmulti-user systems.

There is therefore a need to consider the impact of cooperation oncellular systems, including system-architecture aspects. The downlinkand uplink, separately and in each case, consider several cooperationschemes which result in different architectures. In each case, theimpact on the system operation is considered, with emphasis on ARQ/HARQand scheduling and solutions are proposed.

With the evolution of users' needs for various high quality and datarate services and applications, the capacities of wireless communicationlinks are being exhausted. Single antenna systems are now being found tobe unable to address these needs, and operators are now moving tomultiple antennas systems. Despite their unprecedented achievable datarates, multiple antenna systems do not provide significant gain at farrange or low signal-to-noise ratio (SNR) applications.

Relayed communication seems to address such an issue and is now thefocus of many research activities. Unlike conventional point-to-pointcommunication techniques, relaying introduces a third entity called a“relay” that assists in the communication between the source and thedestination.

When assisting the source, the relay and the source agree to variousprotocols to deliver the intended message to its destination, forexample hopping and diversity protocols. With hopping, the message issent by the source, received by the relay and then retransmitted to thedestination. With diversity protocols, the relay and the sourcesimultaneously transmit to the destination using some diversity schemes.

The versatility introduced by the relay in terms of deployment andproviding additional virtual antennas, are the key advantages ofrelaying systems. For example, multiple antennas are limited in size andcost, and thus are difficult to implement with more than four antennas.However, with relaying, the number of antennas in a link may beincreased in a distributed manner, and thus can introduce higher gainsin data rates. Also, by adjusting relay locations or by selecting theones with the appropriate channel conditions, low SNR and far-rangelinks receive a significant boost. Further, cell edge users aregenerally disfavored due to the high interference they experience.Relaying in this case can be used to increase and redistribute thethroughput throughout the cell and enhance the disfavored links.

Notwithstanding these significant advantages of relaying and theextensive theoretical development in cooperative communication, littlework has been performed towards introducing the benefits of cooperativecommunications to practical cellular systems. Some of the reasons forthis are lack of efficient cooperation protocols with demonstratedbenefits in real world scenarios and expensive implementations.Consequently, there is a need for cooperative communication protocolsthat are appropriate for cellular communications systems.

Relay communications has shown much promise recently in improvingcommunications on weak communications links. By allowing the relay totransmit the full message to the destination in a multi-hop fashion,extremely remote communicating ends have been provided connectivity.However, multiple-hops result in communication delays that may beunacceptable in certain real-time applications.

A more improved structure for relayed communications is cooperativecommunications. Unlike multi-hopping, the source and the relay ormultiple relays collaborate to provide diversity or multiplexing gains.As an example, the source and the relay could transmit in an Alamoutischeme. Relays are provided the option of decoding the message beforehelping or simply forwarding it after adapting its power to the channel.These techniques are called decode and forward (DF) and amplify andforward (AF), respectively.

The main disadvantage of these techniques is that delays are introducedby the relays when DF is assumed. One way to avoid this is to use a formof coding that allows the destination to collect data from the beginningof the communications while relays are receiving. By doing so, delaysdue to the DF protocols are reduced. The destination thus sees acontinuous transmission throughout.

In another scheme, fountain codes, a special case of rateless codesoptimally built for erasure channels, have been used for broadcastapplications. However, there is a need for efficient use of ratelesscoding for practical relay systems.

Due to the propagation delay between the RS and BS, the frequency offsetbetween the BS and RS local oscillators, as well as the processingdelays in the RS, the timing of the RS transmissions to the WTRU may bedifferent from the timing of the BS transmissions to the WTRU. Duringthe cooperation phase, misalignment of the streams received by the WTRUfrom the BS and RS respectively may cause interference with each other.The inter-stream interference reduces the data rate that can be achievedby the WTRU, thus reducing the potential benefit from cooperation.

It would therefore be desirable to mitigate this problem bysynchronizing the BS and the RS DL transmissions. Using synchronized BSand RS DL transmissions would help reduce the interference between theRS and the BS transmissions to the WTRU and enable the use of variousdiversity schemes (e.g. Alamouti or MIMO schemes) while avoiding complexWTRU receiver design.

Prior art solutions show that adjusting the timing of the uplink (UL)WTRU's transmission may be achieved through a timing adjust (TA)mechanism. While the TA concept is commonly used for the UL, so far ithas not been used for the DL which is needed in the context ofcooperative networks.

It would also be desirable to improve link performance through anintelligent use of relays. However, simple multi-hop relaying (i.e. onewhere the relay just forwards the same data that it receives) is notlikely to result in significant gains. Instead, more sophisticatedcooperative techniques may be employed. Among these are cooperativecoding schemes, such as distributed beam-forming and distributed spatialmultiplexing techniques. It would therefore be desirable to use amulti-user detector, more precisely a successive interference canceller(SIC), to optimize the performance of joint reception of transmissionsfrom the source and relay. A minimum mean squared error successiveinterference canceller (MMSE-SIC) receiver is formally a candidatereceiver for use in the Third Generation Partnership Project's (3GPP's)Long Term Evolution (LTE) technology for separating between spatialstreams emanating from the same transmitter. Thus, it would be desirableto place the source and relay transmissions into separate transmissionstreams and use a SIC to receive these transmissions. In fact, at leastfor OFDM MIMO technologies, it may not even require additional receiverstructures.

Specifically, the SIC receiver would be able to take advantage ofapparent practicability and demonstrate that once such a receiver isintroduced into a communication system, much of the advantage ofcooperative diversity may be relegated to the MAC layer. Instead ofcollaborative transmission and coding, a well scheduled combination ofdirect transmission and simple multi-hop would be desirable to achievethe benefits of cooperative relays and, in some cases, even exceed whatcan be delivered by a well designed PHY-layer scheme.

SUMMARY

A method and apparatus for cooperation in wireless communications.Cooperation is considered among a number of network elements, includingat least one wireless transmit-receive unit, at least one relay station,and at least one base station.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the disclosure may be had from thefollowing description of embodiments, given by way of example and to beunderstood in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagram of four relay architectures for use in cellularsystems;

FIG. 2 is a diagram of an example cooperative relay architecture;

FIG. 3 is a diagram of an example multiple-WTRU-serving-relayArchitecture;

FIG. 4 is a diagram of a variation to the forwarding relay architecturewhen multiple relays are wirelessly connected in series;

FIG. 5 is a diagram of multiple cells in a system where the associationbetween the RS and the BS may be static or dynamic;

FIG. 6 is a diagram of an example architecture where the RS may beassociated with more than one BS;

FIG. 7 is a diagram showing example TDM relays that essentially transmitand receive signals in different time intervals;

FIG. 8 is a flow diagram showing the sequence of actions involved in adecode-and-forward scheme;

FIG. 9 is a flow diagram of protocol 1 (P1) which is defined for thedownlink (DL) as follow;

FIG. 10 is a flow diagram showing and example multicast-split RTSreferred to as protocol 2 (P2) and defined as follows for the DL case;

FIG. 11 is a diagram of an example full information relay;

FIG. 12 is a diagram of an example forwarding relay;

FIG. 13 is a diagram of an example cooperative relay;

FIG. 14 a diagram of an example forwarding relay with FDM MIMO;

FIG. 15 a diagram of an example TDDR solution;

FIG. 16 is a diagram of another example embodiment using a fountainextended time division duplex relaying (FTDDR) scheme;

FIG. 17 is a diagram of an example parallel transmission duplex relaying(PTDR) protocol;

FIG. 18 is a diagram of an example STDDR;

FIG. 19 is a diagram of an example protocol stack;

FIG. 20 is a diagram of a second alternative for implementing the WTRUprotocol stack;

FIG. 21 is a diagram of a third alternative for implementing the WTRUprotocol stack;

FIG. 22 is a flow diagram of the sequence of events involved intransferring an IP packet from the BS to the WTRU via the RS;

FIG. 23 is a flow diagram of the sequence of events involved for aMAC-level RS;

FIGS. 24 a and 24 b are diagrams of alternate embodiments for datatransfer using an RLC-level RS;

FIG. 25 is a diagram of operations in the user plane in two-hop mode;

FIG. 26 is an example diagram of a MAC-relay sublayer of the MACsituated between the RS and the BS;

FIG. 27 is diagram of an example protocol architecture for a PHY-levelRS;

FIG. 28 is a diagram of an example protocol architecture for a MAC-levelRS;

FIG. 29 is a diagram of an example protocol architecture for a RLC-levelRS;

FIG. 30 is a diagram of the sequence of events involved in transferringan IP packet from the BS to the WTRU via the RS;

FIG. 31 is a flow diagram of the data transfer operation when the BS isnot aware of the detailed relay operation;

FIG. 32 is a diagram of example signal flows for a smart relay and aslave relay;

FIGS. 33 a and 33 b are diagrams of example protocol architectures wherethe BS and the relay contain a layer 2 contour plane entity;

FIG. 34 is a diagram of an example cooperative header;

FIG. 35 is a diagram of an example technique that may separate channelcoding for the header and the payload;

FIG. 36 is a flow diagram of an example technique used to separatechannel coding for the header and the payload;

FIG. 37 is a diagram of a downlink data packet having a header and apayload;

FIG. 38 is a diagram of a relay system using five channel states;

FIG. 39 is a diagram of a transmission header comprising a “legacy”header appended by 1 bit, called “coop. header indicator bit”;

FIG. 40 is a diagram of an example downlink scheme;

FIG. 41 is a diagram of an example downlink scheme;

FIG. 42 is a diagram of an example downlink scheme;

FIG. 43 is a diagram of an example downlink scheme;

FIG. 44 is a diagram of an example downlink scheme;

FIG. 45 is a diagram of an example downlink scheme;

FIG. 46 is a diagram of an example downlink scheme;

FIG. 47 is a diagram of an example downlink scheme;

FIG. 48 is a diagram of control channels for the DL;

FIG. 49 is a diagram of an example of the control channels for UL;

FIG. 50 is a diagram of an example of the control channels for UL;

FIG. 51 is a diagram of an example frame structure for an SI; and

FIG. 52 is a diagram of an example synchronization of the BS and RS DLtransmissions to the WTRU using a timing adjust procedure.

DETAILED DESCRIPTION

When referred to hereafter, the terminology “wireless transmit/receiveunit (WTRU)” includes but is not limited to a user equipment (UE), amobile station, a fixed or mobile subscriber unit, a pager, a cellulartelephone, a personal digital assistant (PDA), a computer, or any othertype of user device capable of operating in a wireless environment. Whenreferred to hereafter, the terminology “base station” includes but isnot limited to a Node-B, a site controller, an access point (AP), or anyother type of interfacing device capable of operating in a wirelessenvironment. When referred to hereafter, the terminology “relay station”may be referred to as a relay or a RS.

Although this disclosure is described in the context of a ThirdGeneration (3G) cellular wireless system, it should not be construed aslimited to that system, the 3G system serves only as an example.

Relay Physical Architectures

Relays may be used in Cellular Systems in a number of ways. In thissection, 4 major architectures are described and depicted in FIG. 1.These 4 example architectures may be used alone or in any combination.These example architectures are referred to as Architecture-1:forwarding relay architecture 110; Architecture-2:multiple-WTRU-serving-relay architecture 120 (also shown in more detailin FIG. 3); Architecture-3: cooperative relay architecture 130 (alsoshown in more detail in FIG. 2); and Architecture-4:multiple-BS-shared-relay architecture 140. Each example architectureincludes at least one WTRU 150, at least one relay station (RS) 160, anda base station (BS) 195.

Each WTRU 150 may contain a transmitter 165, a receiver 170, and aprocessor 175. Each RS 160 may contain a transmitter 180, a receiver185, and a processor 190.

Various signals may be transmitted and received by the various nodes ineach of these architectures and are described in detail below. In fact,several signaling embodiments may exist per each architecture. We shallrefer to these embodiments collectively as “relay transmission schemes”or simply as “transmission schemes”. The advantages and applications ofthe above 4 architectures are discussed below.

In Architecture-1 110, the WTRU may receive signals only from the RS160, but not directly from the BS 195. In other words, the WTRU 110 isin a deeply shadowed region of the BS coverage or simply in a BScoverage hole. It is also an architecture useful to serve WTRUs at theedge of a cell, where the inter-cell interference from adjacent cellscan be large. In such cases, the forwarding relay 160 receives the DLdata from the BS 195 and simply forwards it to the WTRU 150 and viceversa for UL data. A variation to the forwarding relay architecture isone where there are multiple relays connected (wirelessly) in series.This is depicted in FIG. 4.

In Architecture-2 120, the RS 160 serves multiple WTRUs 150, that arelocated in the coverage region of the RS 160. The advantage of thisarchitecture is that the BS-RS data communication may be pooledtogether, to reduce overhead. For example, the overhead associated withvarious packet headers may be reduced by defining a combined header forthe pooled packets.

In Architecture-3 130, the WTRU 150 may also be able to receive andprocess signals directly from the BS 195, although typically weaker thanthe signals received from the RS 160. Such a configuration has twoimportant implications. First, as the BS 195 is sending DL data to theRS 160, the WTRU 150 may monitor and receive some of the data or all ofthe data with a certain probability of error. This type of data is oftenreferred to as ‘soft’ data. This reduces the amount of data to be‘forwarded’ by the RS 160 or it increases the probability of successfulreception of the data ‘forwarded’ by the RS 160. Second, the BS 195 andthe RS 160 may simultaneously transmit two ‘cooperative’ signals to theWTRU 150, emulating a multiple antenna scenario. Since the ‘multipleantennas’ in this case are not collocated, we refer to this as‘distributed MIMO’ configuration. The advantages of this Architectureare similar to the benefits of using MIMO.

Architecture-4 140 allows multiple RSs 160 to assist a WTRU 150. Thisexample may also be viewed as a distributed-MIMO configuration, with itsconsequent improvements in performance.

Combining two or more of the 4 basic architectures in a technicallystraight-forward manner may yield practical configurations to overcomethe problems discussed above.

In each of the architectures in FIG. 1, RSs 160 are shown to beassociated with the given cell. When there are multiple cells in asystem, this association between the RS 160 and the BS 195 may be staticor dynamic, as shown in FIG. 5. In other embodiments, a RS 160 may beassociated with more than one BS 195, as shown in FIG. 6. This allowsfor coordination among multiple cells and the capability to serve agroup of WTRUs effectively via a set of shared RSs.

Referring to FIG. 1, when a single BS-RS Channel 155 is serving a SingleWTRU 150, a technical issue that needs to be solved is how the WTRUs 150are informed about the RSs 160. For example, this may be done by the BS195 broadcasting information about the RSs 160. Alternately, the RSs 160may broadcast their presence. Another technical issue is how a WTRU 150selects a RS 160 and how it associates itself to the selected RS 160. Anadded complexity is that this association information should also besent to the BS 195.

Regarding synchronization, the delay incurred in the relay adds to thebulk transmission delay between the BS 195 and the WTRU 150. In turn,the round trip time (RTT) is also affected, which may affect theperformance of certain protocols, such as TCP and ARQ. As a consequence,the buffer requirements at the BS 195 and the WTRU 150 may alsoincrease.

A minimum amount of signaling information must be exchanged between theWTRU-relay, BS-relay and WTRU-relay-BS. It must therefore be determinedwhat these signaling needs are and how they are communicated. Forexample, the power control messages and timing advance messages needonly go between the WTRU 150 and the RS 160, and need not be transmittedto the BS 195.

Relay Transmission Schemes (RTSs)

The previous section introduced various physical architectures for usingRSs in a cellular network, noting that each architecture could supportdifferent choices of signals for transmission and reception by differentnodes. This section describes a number of such ‘transmission schemes’and analyzes their performance. FIG. 7 is a diagram of example TDMrelays that transmit and receive signals in different time intervals.For example, in the DL, a TDM-relay 705 a . . . 705 j would receivesignals from the BS 710 in one time interval and transmit it to the WTRUin a subsequent time interval. These time intervals are referred to asphases or transmission time intervals (TTIs), T1 720 and T2 730.Although T1 720 and T2 730 are drawn contiguously in FIG. 7, T2 does notneed to be contiguous to T1. In fact, in some embodiments, T2 730 willprobably not be contiguous to T1 720, possibly due to schedulingconstraints. Furthermore, the durations of T1 720 and T2 730 areflexible and depend upon the channel conditions, which in turn determinethe time taken for successful reception of a given block of data. In oneexample, the transmission medium is slotted into TTIs of fixed size, sothat T1 720 and T2 730 may be integer multiples of a fixed TTI. HoweverTTI size may also be variable or changed dynamically. T1 720 and T2 730may differ in size from each other.

It is also possible to design other types of relays, for example, FDMrelays, which would transmit and receive in different frequency bands.These designs are general and apply to types of relays other thanTDM-relays. For simplicity, only the details of various designs withinthe context of TDM-relays will be discussed. Although the designs arediscussed in the context of DL data transmission, the designs apply toUL data transmission as well.

Referring again to FIG. 7, the basic principle of a TDM-relay 705 isthat it receives the DL data transmitted from the BS 710 during Phase 1,denoted as T1 720 and transmits it in the DL direction to the WTRU 740in Phase 2, denoted as T2 730. These transmissions are referred to as a“simple” Phase 1 transmission and a “forwarding” Phase 2 transmission.It is possible that during Phase 1, the WTRU 740 also receives andattempts to decode the DL data transmitted from the BS 710. This isreferred to as a “multicast” Phase 1 transmission. Similarly, duringPhase 2, it is possible that the BS 710 also transmits DL data to theWTRU 740. This is referred to as a “cooperative” Phase 2 transmission.These variations to Phase 1 and Phase 2 now produce 4 basic TDM-relaytransmission schemes referred to as simple-forwarding relay transmissionscheme, multicast-forwarding relay transmission scheme,simple-cooperative relay transmission scheme, and multicast-cooperativerelay transmission scheme.

In a simple-forwarding relay transmission scheme, the BS may transmit DLdata, using channel codes, including forward-error-correction codes suchas convolutional or turbo or LDPC codes and error-detection codes, suchas CRC-Block codes; modulation schemes such as M-ary QAM etc; andmulti-antenna (MIMO) mapping schemes. The forwarded signal in Phase 2may be based on the received baseband signal, the received demodulatedsignal, or the received decoded data. The resulting schemes are referredto as “amplify-and-forward”, “demodulate-and-forward” and“decode-and-forward,” respectively. In the latter two cases, the newmodulation and/or the new channel code used for forwarding may bedifferent than the modulation and/or channel code used in Phase 1,because the quality of the RS-WTRU link differs from the quality of theBS-RS link.

FIG. 8 is a flow diagram of the sequence of actions involved in adecode-and-forward scheme 800, where the BS selects an RS 810 andtransmits a message for a WTRU to the selected RS 820. The RS decodesthe message and re-encodes it according to a channel quality metric 830.If necessary the BS retransmits the DL data to the RS, until the RSreceives it without any errors (not shown). ARQ and/or HARQ protocolsmay be used for achieving this error-free transmission. In such a case,Phase 1 is essentially defined by the time taken for the RS to correctlyreceive the data sent by the BS. Similarly, during Phase 2, the RStransmits, and possibly retransmits, the DL data until the WTRUcorrectly decodes it 840.

In the multicast-forwarding relay transmission scheme, the signalstransmitted by the BS are received not only by the RS, but also by theWTRU in Phase 1. In Phase 2, the RS forwards the received signals to theWTRU, which the WTRU would ‘combine’ with the BS signals received duringPhase 1 to correctly receive the BS data. The ‘combining’ processenables this relay transmission scheme to outperform thesimple-forwarding scheme. While the RS may forward the received BS datausing one of the three possible forwarding schemes (namelyamplify-and-forward or demodulate-and-forward or decode-and-forward),only the decode-and-forward scheme will be discussed for simplicity. Inthis case, again, the BS transmits and possibly retransmits DL datauntil the RS correctly decodes it, which indicates the end of Phase 1.

The transmitted signal in Phase 1 may be channel coded using forwarderror correction & detection codes, in which case the WTRU will have atthe end of Phase 1, a soft-decoded version of the DL data sent by theBS. Indeed, the WTRU typically will not be able to correctly decode theDL data during Phase 1 (since the poorer BS-WTRU channel degrades the BSsignal more than the BS-RS channel) and has to decode the data withassociated reliability metric (i.e. soft data). During Phase 2, the WTRUsoft-combines the Phase 1 soft-data with the data forwarded by the RSand correctly decodes the RS transmitted data, possibly after someretransmissions.

Alternately, the transmitted signals in Phase 1 may be coded usingrateless-codes. These codes essentially are channel codes that aresuited for use in single transmitter and multiple receiver communicationscenarios. One advantage with these codes is that at the end of Phase 1,while the RS has decoded all the DL data correctly, the WTRU (due topoorer channel conditions) would have decoded only a subset of thecomplete DL data. Since this is ‘hard’ data (i.e. correct withprobability 1), the Phase 2 data transmission by the RS may be limitedto transmitting only the remaining DL data (which was not correctlydecoded by the WTRU) and the WTRU may simply concatenate the DL datadecoded correctly in Phase 1 and Phase 2, avoiding the need for‘soft-combining’. Finally, the transmitted signals may be channel codedusing any of the existing point-to-multipoint optimal channel codes.

In the simple-cooperative relay transmission scheme, Phase 1transmission details are identical to that of simple-forwarding relaytransmission scheme. At the end of Phase 1, the relay has successfullydecoded the DL data transmitted by the BS. In Phase 2, the BS and RS maytransmit signals in a ‘cooperative’ manner and enhance the efficiency ofthe data transfer to the WTRU. There are different ways in which the BSand RS can cooperate, which include diversity transmission of identicalsignals (which can be used for multipath diversity reception),coordinated transmission of signals for beam-forming at the WTRU (whichrequires channel state information at the transmitters), diversitytransmission of distributed space-time coded signals (for example,Alamouti coding), and higher-rate transmission using distributed spatialmultiplexing schemes (for example pre-coding techniques).

The effective data rates that may be achieved by using these varioustransmission schemes is discussed below. To calculate the effective datarates, the achievable rates on each link (BS-RS, RS-WTRU and BS-WTRU)during each of the two phases may be combined to obtain an effectiveachievable rate for each relay transmission scheme. This combined rateis referred to as “effective throughput TP_(eff)”. The achievable ratesfor each link may be understood to be the theoretical informationcapacity rates or SINR vs. Rate curves computed from link-levelsimulations.

TP_(eff) for Simple-Forwarding RTS

This RTS is also referred as ‘2-hop’ scheme for brevity. In thisexample, the BS transmits b information bits to the selected RS until itfully decodes the bits. The RS may then transmit the decoded bits. Onlythen does the WTRU start the decoding process. The effective throughputin this case is illustrated by the equation:

$\begin{matrix}{{{TP}_{eff}\left( {2 - {hop}} \right)} = {\frac{b}{T_{RS} + T_{U}} = \frac{R_{{BS} - {RS}} \times R_{{RS} - U}}{R_{{BS} - {RS}} + R_{{RS} - U}}}} & {{Equation}\mspace{14mu} (1)} \\{where} & \; \\{T_{RS} = {{\frac{b}{R_{{BS} - {RS}}}\mspace{14mu} {and}\mspace{14mu} T_{U}} = {\frac{b}{R_{U}(2)} = {\frac{b}{R_{{RS} - U}}.}}}} & \;\end{matrix}$

TP_(eff) for Multicast-Forwarding RTS

As described above, channel codes and rateless codes may be used for theMulticast-Phase 1. In this example, rateless codes are used. In atheoretical sense, rateless codes are an infinitely long stream ofencoded bits that make the decoding process independent of the channelconditions. The WTRU may begin decoding the DL data being sent from theBS to the RS at the start of the communication by the BS. Hence, theWTRU decodes some of the bits being sent from the BS in the first phasewith a rate R_(U)(1)=R_(BS-U). In the second phase, the RS resumes thetransmission from the BS by sending only the remaining bits that theWTRU did not decode yet at a rate R_(U)(2)=R_(RS-U). It follows that:

$\begin{matrix}{{T_{RS} = \frac{b}{R_{{BS} - {RS}}}};\mspace{14mu} {and}} & {{Equation}\mspace{14mu} (2)} \\{T_{U} = {\frac{b}{R_{{RS} - U}}\left( {1 - \frac{R_{{BS} - U}}{R_{{BS} - {RS}}}} \right)}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

The effective throughput in this example satisfies the equation:

$\begin{matrix}{{{TP}_{eff}({Rateless})} = \frac{1}{\frac{1}{R_{{BS} - {RS}}} + {\frac{1}{R_{{RS} - U}}\left( {1 - \frac{R_{{BS} - U}}{R_{{BS} - {RS}}}} \right)}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

TP_(eff) for Simple-Cooperative RTS

As described earlier, DL cooperative transmission from the BS and RS inPhase 2 may be viewed as a distributed antenna array transmission.Accordingly, this scheme also is referred to as Simple-DAA, (DAAscheme). The DAA scheme enables the BS and the RS to send differentinformation bits to the WTRU simultaneously. Their signals are thusinevitably interfering with each other. The WTRU uses successiveinterference cancellation (SIC) to distinguish between the interferingsignals. Assuming perfect interference cancellation at the WTRU, therate achieved at the WTRU in the second phase satisfies the equation:

R _(U)(2)=R _(BS-U)(2)+R _(RS-U)  Equation (5)

where R_(BS-U)(2) is the transmission rate of the BS in the second phaseand R_(BS-U)(1) is the BS rate in the first phase.

The effective throughput is now given by Equations (1) and (5) as:

$\begin{matrix}{{{TP}_{eff}({DAA})} = \frac{R_{{BS} - {RS}} \times \left( {{R_{{BS} - U}(2)} + R_{{RS} - U}} \right)}{R_{{BS} - {RS}} + {R_{{BS} - U}(2)} + R_{{RS} - U}}} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

TP_(eff) for Multicast-Cooperative RTS

As described above, channel codes and rateless codes may be used forMulticast-Phase 1. In this example, rateless codes are used. The BS andRS will split only the bits that the WTRU did not recover duringPhase 1. The effective throughput TP_(eff)(Rateless-DAA) is derived fromthe equations above as:

$\begin{matrix}{{{TP}_{eff}({Rateless})} = \frac{1}{\frac{1}{R_{{BS} - {RS}}} + {\frac{1}{\left( {{R_{{BS} - U}(2)} + R_{{RS} - U}} \right)}\left( {1 - \frac{R_{{BS} - U}(1)}{R_{{BS} - {RS}}}} \right)}}} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

An alternate, but more detailed, example of this RTS, which is referredto as protocol 1 (P1), will now be described.

FIG. 9 is a flow diagram of protocol 1 (P1) 900 which is defined for thedownlink (DL) as follows. Assuming a message of m bits, the BS 910encodes the m bits at a rate R₁;BS;RS and transmits these in Phase 1.Since the RS 920 must successfully decode all data, m must follow theequation, m≦R_(1,BS,RS)T₁. During T1, the WTRU 930 also receives thesignal and attempts to decode it.

In Phase 2, the BS 910 and RS 920 utilize a distributed space-time codelayered with an incremental redundancy encoding of the data to transmitthe data to the WTRU 930. The WTRU 930 uses its optimal space timedecoder and then combines the two incrementally redundant transmissionsto fully decode the data at the end of Phase 2. The WTRU 930 combinesdata from 2 transmissions to successfully decode the data. In thisexample, R_(1,BS,US) is the maximal rate at which the reliabletransmission from the BS 910 to the WTRU is possible in Phase 1. LetR_(2,COOP) be the maximal rate at which reliable transmission to WTRU930 is possible by cooperation of the RS 920 and the BS 910 in Phase 2.Assuming ideal incremental redundancy combining, the WTRU 930 possessesR_(1,BS,UE)T₁ bits of information about the message from the firsttransmission and R_(2,COOP)T₂ bits of information about the message fromthe second transmission. To successfully decode the data, m musttherefore have m·m≦R_(1,BS,RS)T₁+R_(2,COOP)T_(2,2). The maximum amountof data that can be transmitted during the TTI (time T) is then given by

m*=max min(R _(1,BS,RS) T ₁ ,R _(2,coop) T ₂ +R _(1,BS,UE) T₁)  Equation (8)

To maximize equation (8),

R _(1,BS,RS) T ₁ =R _(2,coop) T ₂ +R _(1,BS,UE) T ₁  Equation (9)

This rate-balancing equation allows determination of both the split ofthe TTI into Phase 1 and Phase 2 and the maximal achievable transmissionrate. The maximal achievable rate is:

$\begin{matrix}{R_{P\; 1} = {\frac{m^{*}}{T} = \frac{R_{1,{BS},{RS}}R_{2,{coop}}}{R_{1,{BS},{RS}} + R_{2,{coop}} - R_{1,{BS},{UE}}}}} & {{Equation}\mspace{14mu} (10)} \\{= {\frac{1}{R_{1,{BS},{RS}}} + {\frac{1}{R_{2,{coop}}}\left( {1 - \frac{R_{1,{BS},{UE}}}{R_{1,{BS},{UE}}}} \right)}}} & {{Equation}\mspace{14mu} (11)}\end{matrix}$

Protocol 1 (P1) is also applicable for Uplink (UL). The UL example issimilar to that shown in FIG. 8, but with the BS 910 and the WTRU 930roles switched (not shown). Protocol 1 (P1) is described next for the ULcase. The WTRU 930 creates a message/packet m. Such message/packet maybe in the form of a MAC protocol data units (PDU), or in any other form.In Phase 1, for example in a first TTI, the WTRU transmits m to the RS920 and the BS 910, using a modulation and coding scheme (MCS) suitablefor the WTRU-RS link. The BS also monitors this transmission in Phase 1.In Phase 2, for example in a later TTI, the WTRU 930 and the RS 920transmit m to the BS 910 using a distributed space-time code, andtransmit a different Incremental Redundancy (IR) version than the onetransmitted in Phase 1.

The BS 910 may use an appropriate receiver, for example an optimal spacetime decoder in Phase 2. Since m may have received multiple IR versionsin Phase 1 and Phase 2, the BS 910 combines the received versions (e.g.Hybrid Automatic Repeat Request (HARQ) combining) in order to improvethe decoding of m.

Split RTS or MAC-Level Cooperative RTS

A split RTS uses the Multicast-Forwarding RTS in conjunction with adirect transmission to improve performance. In any of the 4 RTSsintroduced earlier, a key element that determines the effectivethroughput is the duration of Phase 1 (T1), during which the DL data ismoved from the BS to the RS. In these examples, the effective throughputincreases if the time taken to do so is decreased. In one example of thesplit RTS, the first phase may be shortened such that the BS splits theDL data b bits into two streams of data b_(RS) and b_(BS). The BS willforward only b_(RS) to the RS in Phase 1, and will transmit b_(BS). tothe WTRU in Phase 2, assuming that b=b_(BS)+b_(RS). In this embodiment,the BS will require knowledge ahead of the start of Phase 1 regardinghow the original b bits will be split in 2 portions, b_(RS) and b_(BS)following the multiplexing mode in Phase 2. Splitting the b bits may beexecuted at the MAC level or the PHY level. The original data dedicatedto the WTRU from the beginning may be split according to the channelconditions and to accommodate simultaneous transmissions. Anotherembodiment concatenates two different messages intended for the WTRU andtransmit using b_(RS) and b_(BS) for each in accordance with the channelconstraints. These constraints are translated in terms of b_(RS) tob_(BS) ratio or T₁ to T₂ ratio.

Two variations of the split RTS are possible, depending on whether thePhase 1 data transmission is ‘simple’ (for example using channel coding)or ‘multicast’ (for example, using rateless coding).

In a simple-split RTS embodiment, assuming that the Phase 1 transmissionis ‘simple’ as described above, the BS will transmit b_(RS) bits to theRS in Phase 1 using a coding technique that allows only the RS to decodethe transmitted codeword. The supported rate on the BS-RS link isdenoted by R_(BS-RS) where:

$\begin{matrix}{b_{RS} = {{T_{1} \times R_{{BS} - {RS}}\mspace{14mu} {or}\mspace{14mu} {also}\mspace{14mu} T_{1}} = \frac{b_{RS}}{R_{{BS} - {RS}}}}} & {{Equation}\mspace{14mu} (12)}\end{matrix}$

In Phase 2, the RS will forward the b_(RS) bits successfully decoded tothe WTRU at the rate R_(RS-U). The BS will simultaneously transmitb_(BS) bits with rate R_(BS-U)(2). This provides:

$\begin{matrix}\begin{matrix}{b_{BS} = {T_{2} \times {R_{{BS} - U}(2)}}} \\{b_{RS} = {T_{2} \times R_{{RS} - U}}}\end{matrix} & {{Equation}\mspace{14mu} (13)}\end{matrix}$

Therefore

$\begin{matrix}{T_{2} = {\frac{b_{BS}}{R_{{BS} - U}(2)} = {\frac{b_{RS}}{R_{{RS} - U}}.}}} & {{Equation}\mspace{14mu} (14)}\end{matrix}$

The split in data satisfies

$\begin{matrix}{b_{BS} = {{R_{{BS} - U}(2)}\frac{b_{RS}}{R_{{RS} - U}}}} & {{Equation}\mspace{14mu} (15)}\end{matrix}$

In time, we have

T ₂ =T ₁ ×R _(BS-RS)  Equation (16)

In a multiplexing mode transmission in Phase 2 where there is perfectsuccessive interference cancellation at the WTRU, the overall rateachieved at the WTRU satisfies:

$\begin{matrix}\begin{matrix}{{R_{U}(2)} = {{R_{{BS} - U}(2)} + R_{{RS} - U}}} \\{= {\log_{2}\left( {1 + \frac{{P_{BS}(2)}\left\lbrack {g_{{BS} - U}^{2} + {\alpha \times g_{{RS} - U}^{2}}} \right\rbrack}{N_{0} + I_{U}}} \right)}}\end{matrix} & {{Equation}\mspace{14mu} (17)}\end{matrix}$

The effective throughput achieved at the WTRU may be expressed as

$\begin{matrix}\begin{matrix}{{TP}_{{Conv} - {Split}} = \frac{b_{RS} + b_{BS}}{T_{1} + T_{2}}} \\{= \frac{R_{{BS} - {RS}}\left( {{R_{{BS} - U}(2)} + R_{{RS} - U}} \right)}{R_{{RS} - U} + R_{{BS} - {RS}}}}\end{matrix} & {{Equation}\mspace{14mu} (18)}\end{matrix}$

In a multicast-split RTS embodiment, assuming that the Phase 1transmission is ‘multicast’, the BS may transmit b_(RS) bits to the RSin Phase 1 using a rateless coding technique. In this example, the RSwill be able to fully decode the transmitted message but will alsoenable other receivers to decode some parts of it. b₁ denotes the bitsthat the WTRU is able to intercept and successfully extract from theBS-RS transmission in Phase 1. b₂ denotes the bits that the WTRUreceives in Phase 2, such that b=b₁+b₂.

The supported rate on the BS-RS link is denoted by R_(BS-RS), and theBS-WTRU link rate, R_(BS-U); where

$\begin{matrix}{{b_{RS} = {{T_{1} \times R_{{BS} - {RS}}\mspace{14mu} {and}\mspace{14mu} b_{1}} = {T_{1} \times R_{{BS} - U}}}}\mspace{20mu}} & {{Equation}\mspace{14mu} (19)} \\{{or}\mspace{14mu} {also}} & \; \\{{{T_{1} = \frac{b_{RS}}{R_{{BS} - {RS}}}},{T_{1} = {\frac{b_{1}}{R_{{BS} - {UE}}}\mspace{14mu} {and}}}}\mspace{14mu} {b_{RS} = \frac{b_{1} \times R_{{BS} - {RS}}}{R_{{BS} - {UE}}}}} & {{Equation}\mspace{14mu} (20)}\end{matrix}$

In Phase 2, the BS may transmit b_(BS) bits at rate R_(BS-U)(2), and theRS will simultaneously forward the b_(RS)−b₁ bits to the WTRU at therate R_(RS-U).

$\begin{matrix}\begin{matrix}{b_{BS} = {T_{2} \times {R_{{BS} - U}(2)}}} \\{{b_{RS} - b_{1}} = {T_{2} \times R_{{RS} - U}}} \\{b_{2} = {b_{BS} + b_{RS} - b_{1}}}\end{matrix} & {{Equation}\mspace{14mu} (21)} \\{therefore} & \; \\{{T_{2} = {\frac{b_{BS}}{R_{{BS} - U}(2)}\mspace{14mu} {and}}}\mspace{14mu} {T_{2} = {\frac{b_{RS} - b_{1}}{R_{{RS} - U}} = {b_{RS}\frac{R_{R} - R_{{BS} - U}}{R_{R} \times R_{{RS} - U}}}}}} & \;\end{matrix}$

The split in data satisfies

$\begin{matrix}{b_{BS} = {b_{RS} \times {R_{{BS} - U}(2)}\frac{R_{{BS} - {RS}} - R_{{BS} - U}}{R_{R} \times R_{{RS} - U}}}} & {{Equation}\mspace{14mu} (22)}\end{matrix}$

Which can be translated in time as follows

$\begin{matrix}{T_{2} = {T_{1} \times \frac{R_{{BS} - {RS}} - R_{{BS} - U}}{R_{{RS} - U}}}} & {{Equation}\mspace{14mu} (23)}\end{matrix}$

Assuming a multiplexing mode transmission in Phase 2 and a perfectsuccessive interference cancellation at the receiver, the overall rateachieved at the WTRU satisfies:

$\quad\begin{matrix}\begin{matrix}{{R_{U}(2)} = {{R_{{BS} - U}(2)} + R_{{RS} - U}}} \\{= {\log_{2}\left( {1 + \frac{{P_{BS}(2)}\left\lbrack {g_{{BS} - U}^{2} + {\alpha \times g_{{RS} - U}^{2}}} \right\rbrack}{N_{0} + I_{U}}} \right)}}\end{matrix} & {{Equation}\mspace{14mu} (24)}\end{matrix}$

and, the effective throughput achieved at the WTRU can be expressed as:

$\begin{matrix}\begin{matrix}{{TP}_{Rateless\_ Split} = \frac{b_{1} + b_{2}}{T_{1} + T_{2}}} \\{= \frac{\begin{matrix}{{R_{{BS} - {RS}}\begin{pmatrix}{{R_{{BS} - U}(2)} +} \\R_{{RS} - U}\end{pmatrix}} -} \\{{R_{{BS} - U}(2)}R_{{BS} - U}}\end{matrix}}{R_{{RS} - U} + R_{{BS} - {RS}} - R_{{BS} - U}}}\end{matrix} & {{Equation}\mspace{14mu} (25)}\end{matrix}$

Data Flow Analysis of Multicast-Split RTS

FIG. 10 is a flow diagram showing a multicast-split RTS embodimentreferred to as protocol 2 (P2) 1000 and defined as follows for the DLcase. The BS 1010 creates two messages of m₁ and m₂ bits. In Phase 1,the BS 1010 transmits the first message (m₁ bits) to the RS 1020 at arate R_(1,BS,RS), thus m₁≦R_(1,BS,RS)T₁. As in P1, the WTRU 1030monitors to this transmission. In Phase 2, the RS 1020 forwards theinformation it received in Phase 1 to the WTRU 1030. This is performedat a rate R_(2,RS,UE). The BS 1010 simultaneously sends the secondmessage (m₂ bits) to the WTRU 1030. This is performed at a rateR_(2,BS,UE). The WTRU 1030 uses a multi-user detector (not shown), forexample a SIC, in Phase 2 and in conjunction with an incrementalredundancy for the first message to receive the data. To analyze theperformance of this protocol, various constraints exist. First, as forP1, transmitting the first message efficiently may occur in accordancewith the following rate-balancing equation:

R _(1,BS,RS) T ₁ =R _(2RS,UE) T ₂ +R _(1,BS,UE) T ₁  Equation (26)

The rates R_(2,RS,UE) and R_(2,BS,UE) are, however, dependent on eachother as well. In addition to satisfying individual per-link capacityconstraints, the rates must also satisfy the MAC capacity constraint:

R _(2,RS,UE) +R _(2,BS,UE) ≦R _(2,coop)  Equation (27)

The assumed rate R_(2;COOP) as defined for P1 is the optimal transmittercooperation rate. Although cooperation at the PHY layer is not part ofP2 (see above), the close relationship between achievable throughput forP1 and P2 is shown herein. Clearly, maximizing the throughput wouldrequire equation (27) to be satisfied with equality. Accordingly,together with equation (26) and the constraint T=T₁+T₂:

$\begin{matrix}{R_{P\; 2} = {\frac{m_{1} + m_{2}}{T} = \frac{{R_{1,{BS},{RS}}R_{2,{coop}}} - {R_{1,{BS},{UE}}R_{2,{BE},{UE}}}}{R_{1,{BS},{RS}} + R_{2,{RS},{UE}} - R_{1,{BS},{UE}}}}} & {{Equation}\mspace{14mu} (28)}\end{matrix}$

In interference limited cellular deployments, P2 provides slightlybetter performance than P1. Both provide a significant improvement overa no-relay case or a simple 2-hop relaying and P2 performs better thanP1. The key difference is in the management of cooperation. In P1, asingle flow is transmitted by the MAC during (T1+T2), while P2 createsand transmits 2 MAC flows.

In order to schedule the data, the MAC is aware of the quality of acompound link comprises the three constituent PHY links (BS-to-RS,RS-to-WTRU and BS-to-WTRU). Moreover, to ensure cooperation between theBS 1010 and the RS 1020 in Phase 2, the RS 1020 must be centrallyscheduled by the BS 1010 and the PHY at the BS 1010 and the RS 1020 mustbe tightly synchronized to the channel symbol level.

P2 manages the transmission of the two flows almost independently andwithout tight PHY layer synchronization. A constraint on the two flowsis that the sum rate at the WTRU 1030 does not exceed its sum rateconstraint. Provided this constraint is satisfied, the BS MAC 1040manages the RS transmission only in a limited fashion. The BS MAC 1040schedules data to the RS 1020 (based only on the BS-to-RS link quality)to ensure that the RS buffer (not shown) does not become empty. The BSand RS MACs 1040 (RS MAC not shown) must agree on how the rates arerepartitioned in Phase 2 such that the combined rate to the WTRU 1030does not violate the sum rate constraint. However, the BS MAC 1040 doesnot need to specify to the RS MAC (not shown) which particular packet isto be scheduled for the transmission. Once the RS 1020 indicatesreception of a packet, HARQ management for that packet may berelinquished to the RS.

The RS MAC scheduler (not shown) may act independently from the BS MACscheduler (not shown) such that the BS 1010 control of the RS 1020occurs at a slower rate. The PHY layer operations of P2 require nocoordination since the BS 1010 and RS 1020 simply transmit differentflows in Phase 2 in a non-cooperative fashion.

P2 is also applicable to the UL. The UL scenario is similar to thatshown in FIG. 10, but with the BS 1010 and the WTRU 1030 roles switched(not shown). P2 is described herein for the UL case.

In this embodiment, the WTRU 1030 creates any two messages/packets m1and m2. m1 and m2 may be created at different times. These two messagesmay be in the form of 2 MAC PDUs, or in any other form. In Phase 1, forexample in a first TTI, the WTRU 1030 transmits m1 to the RS 1020 andthe BS 1010 using an MCS suitable for the WTRU-RS link. The BS 1010 alsomonitors this transmission in Phase 1. In Phase 2, for example in alater TTI, the RS 1020 forwards the information it received in Phase 1to the BS 1010 using an MCS suitable for the RS-BS link, andtransmitting a different IR version than the one it received from theWTRU 1030. In Phase 2, for example in a later TTI, the WTRU 1030 alsosends a second message m2 to the BS 1010 using an MCS suitable for theWTRU-BS link.

The BS 1010 may use an appropriate receiver, for example a multi-userdetector or a SIC (not shown), in Phase 2 to receive m1 and m2. Sincesome messages, such as m1, may have received multiple IR versions (e.g.in Phase 1 and Phase 2), the BS 1010 combines the received versions(e.g. HARQ combining) in order to improve the decoding of the message.

Relay Transmission Schemes for OFDM-Like Systems

The frequency dimension may be exploited in cooperative relay schemes.Although the following example embodiments apply to the DL, only the ULis discussed for simplicity.

The frequency band assigned for transmission between the WTRU and the BSare divided into two frequency bands, W11 and W12. W11 is used fortransmission from the WTRU to the BS and W12 is used for transmissionfrom the WTRU to the RS. Generally, the WTRU may use differentsubcarriers to transmit the data to different receivers, RSs and BSs.This embodiment assumes that the channel between the WTRU and the relaystation is better than the one between the WTRU and the BS. For the fulland partial information relay examples below, it is assumed that therelay is operated in TDM mode, which implies that the relay cannotreceive and transmit the signal at the same time. For the continuoustransmission example, it is assumed that the relay is operated in FDMmode, which implies that the relay cannot receive and transmit thesignal in the same frequency band.

Full Information Relay

FIG. 11 is an example diagram of a full information relay 1100embodiment. In this embodiment, the RS 1110 has full information duringan uplink transmission, and the following describes a sequence ofsignaling between the WTRU 1120, RS 1110 and BS 1130.

The WTRU 1120 transmits the packet to the RS 1110 and the BS 1130 at thesame time but in different frequencies (f₁ and f₂) 1121,1122 and the RS1110 will get the packet correctly before the BS 1130 does. After the RS1110 receives the signal successfully from the WTRU 1120, the RS 1110sends an ACK 1125 to the WTRU 1120. The BS 1130 now has obtained b1 bitscorrectly from the WTRU's 1120 direct transmission 1135. There are threeoptions for transmitting the remaining b2 bits.

The first option is the forwarding relay 1140. In this option, the WTRU1120 stops transmission in W11 and the RS 1110 forwards b2 bits to theBS 1130 using W11 and W12 1142 until it receives an ACK 1145 from the BS1130. The advantages under this option include power saving in the WTRU1120 since the WTRU 1120 only transmits its packet after the RS 1110successfully receives the transmission resulting in less signalingrequired in the WTRU 1120. The required signaling includes an ACK 1125from the RS 1110 to the WTRU 1120, and an ACK 1145 from the BS 1130 tothe RS 1110. However, the BS 1130 needs to be informed that thetransmission is coming from the RS 1110 after the RS 1110 sends an ACK1125 back to the WTRU 1120. The effective rate is as follows:

$\begin{matrix}\begin{matrix}{T_{1} = \frac{b}{R_{{UE} - {RS}}}} \\{T_{2} = \frac{b - {R_{{UE} - {BS}}T_{1}}}{R_{{RS} - {BS}}}} \\{{TP}_{eff} = {\frac{b}{T_{1} + T_{2}} = \frac{R_{{UE} - {RS}}R_{{RS} - {BS}}}{R_{{UE} - {RS}} + R_{{RS} - {BS}} - R_{{UE} - {BS}}}}}\end{matrix} & {{Equation}\mspace{14mu} (29)} \\{where} & \mspace{11mu} \\\begin{matrix}{R_{{UE} - {RS}} = {\frac{W_{12}}{2}{\log_{2}\left( {1 + \frac{{SINR}_{{UE} - {RS}}}{W_{12}}} \right)}}} \\{R_{{UE} - {BS}} = {\frac{W_{11}}{2}{\log_{2}\left( {1 + \frac{{SINR}_{{UE} - {BS}}}{W_{11}}} \right)}}} \\{R_{{RS} - {BS}} = {\frac{W_{1}}{2}{\log_{2}\left( {1 + \frac{{SINR}_{{RS} - {BS}}}{W_{1}}} \right)}}}\end{matrix} & \;\end{matrix}$

The second option is a cooperative relay 1150. In this option, the WTRU1120 does not stop transmission in W12. The WTRU 1120 and the RS 1110coordinately transmit b2 bits to the BS in W11 1152 and W12 1155respectively using either predetermined Distributed MIMO mode orpredetermined Cooperative Diversity. Note that “predetermined” in thisexample means that no signaling is required between the RS and the WTRUregarding how distribution or cooperative diversity is performed.

The advantage of this option is that it requires a shorter time forsuccessful transmission, hence, resulting in a possibly higher effectiverate compared with the first option above. To achieve this highereffective rate however, more power consumption in the WTRU 1120 isrequired compared with the first option and more signaling is requiredfor an ACK 1125 from the RS 1110 to the WTRU 1120, synchronizationbetween the RS 1110 and the WTRU 1120, an ACK 1145 from the BS 1130 tothe RS 1110 and the WTRU 1120, the BS 1130 needs to be informed that thetransmission is coming from the WTRU 1120 and the RS 1110, and the BS1130 needs to be informed of the transmission mode as well.

The effective rate is as follows. For cooperative diversity in Phase 2,the RS 1110 and the WTRU 1120 cooperatively transmit the same bits (b2bits) to the BS but use different frequency bands such that theincreased frequency diversity strengthens the reliability ofcommunications of b2 bits. For distributed MIMO in Phase 2, the RS 1110and the WTRU 1120 independently transmit different bits to the BS andthe total number of bits transmitted by the RS 1110 and the WTRU 1120 isb2.

$\quad\begin{matrix}\begin{matrix}{T_{1} = \frac{b}{R_{{UE} - {RS}}}} \\{T_{2} = \frac{b - b_{1}}{{R_{{UE} - {BS}}(2)} + R_{{RS} - {BS}}}} \\{b_{1} = {T_{1}{R_{{UE} - {BS}}(1)}}} \\{{R_{eff}({full})} = {\frac{b}{T_{1} + T_{2}} = \frac{R_{{UE} - {RS}}\begin{pmatrix}{{R_{{UE} - {BS}}(2)} +} \\R_{{RS} - {BS}}\end{pmatrix}}{{R_{{UE} - {BS}}(2)} + R_{{RS} - {BS}} + R_{{UE} - {RS}} - {R_{{UE} - {BS}}(1)}}}}\end{matrix} & {{Equation}\mspace{14mu} (30)}\end{matrix}$

The third option is a forwarding relay with FDM MIMO 1160. In this thirdoption, the RS 1110 forwards all the remaining bits (b2 bits) 1162 tothe BS 1130 in W12 until it receives an ACK 1165 from the BS 1130 andthe WTRU 1120 starts a new transmission in W11 1167. During thisduration, there are b′ bits successfully transmitted 1169 from the WTRU1120 and the BS 1130.

The advantages of this option include that it is an efficienttransmission, hence, resulting in a possibly higher overall throughputcompared with the first and second options above. However, moresignaling is required for an ACK from the RS to the WTRU and an ACK fromthe BS to the RS, the BS needs to be informed that the RS forwards theremaining bits and the WTRU starts a new transmission after RS sends ACKback to WTRU. To maximize the throughput, the bandwidth allocation inPhase 1 and Phase 2 could be different.

Partial Information in Relay

For the previously described embodiment regarding a relay with fullinformation, several schemes may be used to describe how a relay may beused in FDM mode to help the WTRU transmit the information to the BS. Inthese schemes, before the RS starts to relay the information to the BS(this duration is defined as Phase 1), only an ACK is required to besent from the RS to the WTRU after the RS succeeds in receiving all thebits from the WTRU, which reduces the signaling overhead. In thisembodiment however, the WTRU sends all the bits to the RS, some of whichare redundant as the WTRU has sent some bits to the BS in Phase 1. Tosave power in the WTRU, it would be more efficient to avoid transmittingthose bits to the RS which have been sent by the WTRU to the BS in Phase1.

Hence, in this embodiment, some examples are described in which the RSreceives some bits from the WTRU before it starts to forward the bits tothe BS. In FIGS. 12-14, the WTRU transmits b1 bits 1210 and b2 bits 1220in different frequencies (f11 and f12) to the BS and the RSrespectively. Proper design of bits allocation/bandwidth allocationbetween transmissions WTRU-RS and WTRU-BS is such that the BS and the RSsuccessfully detect their bits at the same time. After the RS receivesthe b2 bits successfully, similar to the full information relayembodiment, there are three options for transmitting the b2 bits fromthe RS to the BS.

FIG. 12 is a diagram of an example forwarding relay 1200. After the WTRUtransmits b1 bits 1210 and b2 bits 1220 to the BS and the RSrespectively, the RS forwards the b2 bits to the BS 1230 in Phase 2.

FIG. 13 is a diagram of an example cooperative relay 1300. Usingcooperative diversity in Phase 2, the RS and the WTRU cooperativelytransmit the same bits (b2 bits) 1310 to the BS, but use differentfrequency bands such that the frequency diversity is increased. Usingcooperative MIMO in Phase 2, the RS and the WTRU independently transmitdifferent bits to the BS and the total number of bits transmitted by theRS and the WTRU is b2. The effective rate for cooperative MIMO is asfollows:

$T_{1} = {\frac{b_{2}}{R_{{UE} - {RS}}} = \frac{b_{1}}{R_{{UE} - {BS}}(1)}}$$T_{2} = \frac{b_{2}}{{R_{{UE} - {BS}}(2)} + R_{{RS} - {BS}}}$

Therefore,

$\begin{matrix}{{b_{1} = {{T_{1}{R_{{UE} - {BS}}(1)}} = {\frac{b_{2}}{R_{{UE} - {RS}}}{R_{{UE} - {BS}}(1)}}}}\begin{matrix}{{R_{eff}\left( {{full} - {MAC}} \right)} = \frac{b_{1} + b_{2}}{T_{1} + T_{2}}} \\{= \frac{\begin{pmatrix}{{R_{{UE} - {BS}}(1)} +} \\R_{{UE} - {RS}}\end{pmatrix}\begin{pmatrix}{R_{{RS} - {BS}} +} \\{R_{{UE} - {BS}}(2)}\end{pmatrix}}{R_{{RS} - {BS}} + {R_{{UE} - {BS}}(2)} + R_{{UE} - {BS}}}}\end{matrix}} & {{Equation}\mspace{14mu} (31)}\end{matrix}$

FIG. 14 is a diagram of an example forwarding relay with FDM MIMO 1400.After the WTRU transmits b1 bits 1210 and b2 bits to the BS and the RSrespectively, the WTRU transmits b′ bits to the BS 1410 and the RStransmits b2 bits to the BS 1420.

To maximize the throughput, the bandwidth allocation in Phase 1 andPhase 2 could be different.

Continuous Transmission

In this section, it is assumed that the RS is a forwarding relayoperated in FDM mode. The RS and the WTRU continuously transmit thepacket to the BS, making full use of radio resource. All nodes transmitall the time—i.e. no TDM. Yet, Phase 1 and Phase 2 are distinguished todistinguish the BS transmission and the RS assistance phases. Table 1shows WTRU and RS actions in continuous transmission.

TABLE 1 Time RS Action WTRU Action T11 (i.e. Phase 1 of N/A WTRUtransmits B1 (i.e. Timeslot 1) data block 1) on f11 T12 (i.e. Phase 2 ofRS forwards B1 on f12 WTRU transmits new Timeslot 1) data block, B2, onf11 T21 (Phase 1 of Timeslot 2) RS forwards B2 on f12 WTRU transmits newdata block, B3, on f11 T22 (Phase 2 of Timeslot 2) RS forwards B3 on f12WTRU transmits new data block, B4, on f11 TN1 (Phase 1 of Timeslot N) RSforwards B2 on f12 WTRU transmits new data block, B(N + 1), on f11 TN2(Phase 2 of Timeslot N) RS forwards B(N + 1) on WTRU transmits new f12data block, B(N + 2), on f11

As a consequence, the WTRU is continually sending data on f11 and the RSis always forwarding the data on f12. Just as time is split into 2phases for the WTRU and the RS transmissions, here frequencies are splitinto 2 segments for the WTRU and the RS transmissions.

Relay Transmission Schemes for Multiple WTRUs

In the following example RTSs, a cell may contain one or more WTRUs andone or more RSs. Depending upon the condition of the channel between aWTRU and the BS and RSs, a direct transmission scheme between the BS andRS or a specific RTS involving one or more RSs may be optimal. In thissection, a number of protocols are discussed for serving multiple WTRUsin a cell with multiple RSs using one of the RTSs described earlier.Three fundamentally different methods are described below.

The first method (Method-1) is referred to as TDDR & FTDDR. In thisexample, the BS serves different WTRUs in different time ‘slots’. In theTDDR, Phase 1 DL transmissions are received only by the RS. In FTDDR,the Phase 1 DL transmissions are received by the RS as well as the WTRU.See FIGS. 15 and 16.

The second method (Method-2) is referred to as PTDDR. In this example,the BS may serve multiple WTRUs in Phase 2 of the TDM-relay operationscheme. See FIG. 18.

The third method (Method-3) is referred to as STDDR and FSTDDR. In thisexample, the BS serves multiple WTRUs in both Phase 1 and Phase 2 of TDMrelay Schemes. FIG. 19 is an example diagram for STDDR 1900. FSTDDR isdescribed later.

TDDR Solutions

FIG. 15 is a diagram of an example TDDR solution 1500. Given L relays ina cell, there are a number of options available in getting the data tothe WTRU. The BS may communicate to the WTRU directly 1510.Alternatively, the BS may select a particular relay, send the data tothis relay and allow the relay to forward the data 1520. Finally, thismethod may involve several relays simultaneously, in which case they allact identically, as a distributed multi input multi output (MIMO)antenna array in the relaying stage. Whether a particular WTRU istransmitted to directly, through a single relay or a through a set ofrelays, depends on, for example, the availability of relays, thelocation of the WTRU, and the relative channel qualities between theWTRU and relative transmitters.

A decision protocol on how to transmit to a particular WTRU may be basedon maximizing the resulting throughput to the WTRU as follows. Forexample, let R_(B-R) denote the data rate achievable between the BS andthe RS, R_(R-U) denotes the data rate achievable between the RS and theWTRU, and R_(B-U) denotes the data rate achievable between the BS andthe WTRU.

In the case when the WTRU is transmitted to directly by the BS, itsthroughput is computed directly:

TP_(B-U) ^(R)=R_(B-U)  Equation (32)

In the case when a single relay is utilized, the throughput is computedas follows:

$\begin{matrix}{{TP}_{B - R - U}^{R} = {\frac{R_{R - U}T_{2}}{T_{2} + T_{1}} = \frac{R_{R - U} \cdot R_{B - R}}{R_{R - U} + R_{B - R}}}} & {{Equation}\mspace{14mu} (33)}\end{matrix}$

where Equation (22) is determined by discounting the relay-to-WTRUthroughput by taking into account the time needed to get data to therelay and balancing the two portions. Values T₁ and T₂ denote timedurations corresponding to R_(R-U) and R_(B-R), respectively.

In the case of multiple relays there are several options. In one option,the multiple relays may be treated as a single multiple-antenna relayand the rates R_(B-R) and R_(R-U) are computed accordingly. In anotheroption, the throughput may be computed for each relay as in Equation(33) then added for the group. Equation (33) is determined bydiscounting the relay-to-WTRU throughput by taking into account the timeneeded to get data to the relay and balancing the two portions.

The transmission direction decision is then made as follows (forsimplicity we ignore the case of “grouped relays”, but the extension isobvious). Whether the WTRU is served by the BS directly or served by theBS via the RS depends on discounted throughput TP_(B-Rl-U) ^(R)lε{1, 2,. . . , L}, where L is the number of RSs, and direct link throughputTP_(B-U) ^(R). Maximum throughput among these throughputs is denoted asTP_(E2E) ^(R)=max(TP_(B-U) ^(R),TP_(B-Rl-U) ^(R), . . . , TP_(B-RL-U)^(R)), which is the end-to-end throughput that may be achieved. That is,the station that could achieve the highest throughput will be servingthe WTRU directly. For multiple WTRU scenarios, TP_(E2E) ^(R) is adoptedas the input of the scheduler.

Scheduler

Whether or not a WTRU is transmitted to in a particular TTI, depends onthe scheduling function (e.g. the HSDPA scheduler in the BS). Thescheduling function may use a decision variable as an input (e.g.,TP_(E2E) ^(R)), as computed above, as well as other inputs, like bufferoccupancy, fairness options, etc.

In accordance with the described protocol, the decision variable isbased on the channel quality conditions (i.e., R_(R-U), R_(B-R), R_(B-R)are computed based on channel conditions) for HARQ scheduling. A typicalHARQ scheduler is then used to determine scheduling.

Feedback

Channel state may be reported to the BSs using feedback (e.g., CQI usingHSDPA). It should be noted that the quality of the relay-to-WTRU channelmay be reported to the BS directly from the WTRU or by the relay. Whenreported by the relays, feedback from multiple WTRUs may be aggregatedinto a single transmission.

The ACK/NACK may be sent directly or forwarded via the relay by uplink,in which case different ACK/NACKs may or may not be aggregated acrossthe WTRUs and the TTIs.

FTDDR Solutions

FIG. 16 is a diagram of another example embodiment using a fountainextended time division duplexed relaying (FTDDR) scheme 1600. In allprevious schemes, the WTRUs to be communicated through a relay must waitfor that relay to start its transmission in order for them to startcollecting useful data. Delays are thus introduced and throughput gainsare reduced. One way to overcome such an issue is to use fountainencoding at each transmitter.

Fountain codes refer to a type of code capable of driving the outageprobability to zero without channel state information at the source. Thetransmitter encodes data into infinite length code stream (in packets),like a fountain that produces an endless supply of water drops. Thereceiver collects the information until it recovers the data perfectly,similar to holding a bucket under a fountain to collect drops until thebucket is full. One of the points regarding fountain codes is that thesource data may be recovered from any set of sufficiently encodedpackets.

A relaying protocol based on fountain codes and TDD is shown in FIG. 16.Data is transmitted from the BS to the WTRU directly 1610 or with thehelp of the RS 1620 depending on whether R_(B-Rl) ^(F)>R_(B-U) ^(F).Here R_(B-Rl) ^(F) denotes the data rate achievable between BS and RS,and R_(B-U) ^(F) denotes the data rate achievable between the BS and theWTRU directly. The notation [F] at the upper right corner indicates thatthis is for a system with FTDDR. If R_(B-Rl) ^(F)≦R_(B-U) ^(F), thenTP_(l) ^(F)≦R_(B-U) ^(F), where TP_(l) ^(F) indicating the throughputachievable between the BS and the WTRU. In this case, all the data istransmitted to the WTRU directly without RS's help. If R_(B-Rl)^(F)>R_(B-U) ^(F), then

$\begin{matrix}\left\{ \begin{matrix}{{{TP}_{U\; 2}^{S} = \frac{R_{B - R}^{S\; 1}T_{1}}{T_{1} + T_{2}}}} \\{{{R_{B - R}^{S\; 1}T_{1}} = {R_{R - {U\; 2}}^{S\; 2}{T_{2}.}}}}\end{matrix} \right. & {{Equation}\mspace{14mu} (36)}\end{matrix}$

In this case, in T₁ ^(l), the BS broadcasts information to both the RSand the WTRU, and the RS receives some “new” information as R_(B-Rl)^(F)>R_(B-U) ^(F). In T₂ ^(l) the RS relays the “new” information to theWTRU. The term “new” here means the information broadcasted from the BSis received by the RS, but not received by the WTRU in T₁ ^(l).Maximizing over a selection of all L RSs, T_(E2E) ^(F)=max(TP_(l) ^(F),. . . , TP_(L) ^(F)), is obtained, where T_(E2E) ^(F) is the throughputachievable with the FTDDR scheme.

In accordance with the HARQ scheduling protocol, unlike previousschemes, the BS does not dedicate relays before hand. Relays in thismethod only send an ACK to the BS. Once received, the first relay ACKingmay be selected for scheduling, or the BS will allow a time observationframe to collect enough relay ACKs and select between them according toa chosen criterion.

If more then one relay is selected, the relays may be scheduled usingcellular schedulers. If more then one WTRU will be served by one relay,data to those WTRUs may be “pooled” into a single transmission, orscheduled separately. Scheduling between the WTRUs may be accomplishedusing similar types of schedulers as a regular cellular system, e.g.,HSDPA. In accordance with this method, channel state feedback is notrequired with FTDDR, since the codes used are rateless.

Relays need to send only an ACK to the BS to allow further scheduling.WTRU ACK/NACKs need to be available at the BS only. This may be sentdirectly or forwarded via the relay by uplink, in which cases differentACK/NACKs may or may not be aggregated across WTRUs and TTIs.

PTDR Solutions

FIG. 17 is an example diagram showing a parallel transmission 1700duplexed relaying (PTDR) protocol. In accordance with this protocol, acell has L relays. For each WTRU, one of the following L+1 transmissionoptions are selected: transmit through relay L or transmit directly froma BS. In this example, the scheduling is based on the channelconditions, as expressed, for example in the achievable rate to the WTRUand may be changed periodically.

In each TTI, a transmission is partitioned into 2 sub-TTIs (phases).During Phase 1 1710, the BS transmits to the relays. These transmissionsinclude information that the relays must deliver to the WTRUs. DuringPhase 2 1720, the relays and the BS transmit (simultaneously) to theWTRUs. It should be noted that in each phase not all the WTRUs (orrelays) may be scheduled. The scheduling within each phase and at eachtransmitter (BS/ready for Phase 2) is performed according to ascheduling process, such as in the current HSDPA, downlink LTE, etc. Thedecision on how to transmit to a particular WTRU may be based onmaximizing the resulting throughput to the WTRU which may be computed asabove i.e., in Equations (33) or (34), or through a discounting formulacustomized for this particular protocol.

Unlike TDDR, HARQ scheduling may be conducted independently by the basestation and by each relay. In Phase 1 1710, the base station schedulestransmissions to relays as follows. WTRU data for WTRUs associated withthe same relay are “pooled” into a single transmission, or scheduledseparately. Scheduling is performed using the same types of schedulersas for a regular cellular system, e.g. HSDPA. In Phase 2 1720, eachrelay and BS independently schedules transmissions of data to the WTRUas in a regular protocol, e.g. HSDPA.

Channel state information is reported back to the base stations usingfeedback, as in current systems (e.g. CQI using HSDPA). The relayhowever, must also be aware of the quality of the relay-to-WTRU channelin order to perform its own independent scheduling. Therefore, while thequality of the relay-to-WTRU channel may be reported to the BS directlyby the WTRU, a reporting via the relay is preferable. When reported bythe relays, feedback from multiple WTRUs may be aggregated into a singletransmission.

Phase 1 and Phase 2 require separate ACK/NACK processes (relay-to-BS inPhase 1) and (WTRU-to-transmitter (relay or BS) in Phase 2).Accordingly, each operates independently in accordance with itsrespective operation in relay-less HARQ systems. Depending on thestructure of control and signaling protocols, the relay may or may notneed to forward its WTRU's ACK/NACK back to the BS.

STDDR Solutions

FIG. 18 is an example timing diagram for a superposition time divisionduplexed relaying (STDDR) relay scheme 1800. This method is an extensionof the PTDDR protocol described above. In accordance with this STDDRprotocol, the BS 1810 schedules different WTRUs 1820 based on theirneeds and channel conditions. In this example, these schedules arecommunicated to the WTRU 1820 either directly or through a relay 1830.As described above, the TTIs are split in two phases. In Phase 1 1840,the BS 1810 simultaneously transmits to the relays and to the WTRUs 1820directly using superposition coding. Resources, such as power, may beshared either equally or according to known power allocation algorithms.In the second phase 1850, relays 1830 may take over part of thecommunication to forward the data they received to their intended WTRUs1820, while the BS 1810 continues to serve those WTRUs 1820 scheduled onthe direct link at full power.

In accordance with this method, let R_(B-R) ^(S1) denote the achievabledata rate between BS and RS, and R_(B-U1) ^(S1) denote the achievabledata rate between BS and WTRU1 in Phase 1. In Phase 2, let R_(B-U1)^(S2) denote the achievable data rate between the BS and WTRU1, andR_(B-U2) ^(S2) denote the achievable data rate between the RS and WTRU2.Thus, for WTRU1,

$\begin{matrix}\left\{ {\begin{matrix}{{{TP}_{{UE}\; 1}^{S} = \frac{{R_{B - {{UE}\; 1}}^{S\; 1}T_{1}} + {R_{B - {{UE}\; 1}}^{S\; 2}T_{2}}}{T_{1} + T_{2}}}} \\{{{R_{B - R}^{S\; 1}T_{1}} = {R_{R - {U\; 2}}^{S\; 2}T_{2}}}}\end{matrix};} \right. & {{Equation}\mspace{14mu} (35)}\end{matrix}$

and for WTRU2,

$\begin{matrix}\left\{ \begin{matrix}{{{TP}_{U\; 2}^{S} = \frac{R_{B - R}^{S\; 1}T_{1}}{T_{1} + T_{2}}}} \\{{{R_{B - R}^{S\; 1}T_{1}} = {R_{R - {U\; 2}}^{S\; 2}{T_{2}.}}}}\end{matrix} \right. & {{Equation}\mspace{14mu} (36)}\end{matrix}$

FSTDDR Solutions

In another embodiment, a fountain & superposition coding time divisionduplexed relaying (FSTDDR) protocol may be used. In accordance with thismethod, FTDDR and STDDR are combined. By doing so, the WTRUs that do notneed to be communicated through a relay are scheduled transmission atthe start of the communication and therefore, do not wait for the relaysto complete servicing. In addition, all data streams are fountain-typeencoded, thus avoiding feedbacks.

Accordingly, assuming L relays, M WTRUs may be communicated throughthese relays, and N other WTRUs may be communicated directly.Communication in each TTI is performed in 2 phases as with STDDR.Communication through each relay is performed as in FTDDR. Thus, inPhase 1, relays and the N WTRUs are scheduled simultaneously, assuming apower sharing scenario. In Phase 2, relays schedule and serve the MWTRUs, while the other N WTRUs continue to be serviced by the BS at ahigher power level.

For HARQ scheduling, communication through relays is scheduled as inFTDDR example discussed above. Direct communication is scheduled as atypical cellular system, e.g., HSDPA, and relays and WTRUs may beserviced on direct link are scheduled as in STDDR. Channel statefeedback is not required with FTDDR, since the codes used are rateless.For ACK/NACK delivery, relays send only an ACK to the BS to allowfurther scheduling. The ACK/NACK is available at the BS only, which maybe sent directly or forwarded via the relay by uplink, in which casesdifferent ACK/NACKs may or may not be aggregated across WTRUs and TTIs.

Relay Protocol Architectures

The following example operations in the user plane may be used in singlecell-single relay cooperation in both two-hop mode and diversity mode.In the two-hop mode a dedicated BS-RS channel may be used. Assume asingle cell (i.e. single BS), M relays and multiple WTRUs. The relaysare designed to improve the link quality between base station (BS) andusers (WTRUs). Each WTRU is served by a single relay.

The BS treats the relay as a WTRU and communicates with it. The relay,on the other hand, acts as a BS towards the WTRU and conducts thecommunication. In order to describe the next level of communicationactions, it is necessary to assume the protocol layers supported by therelay.

Consider the BS-side of the RS. Since this side simulates the WTRU,there are choices in terms of how much of the WTRU protocol stack isimplemented. Following are various alternatives.

FIG. 19 shows a first alternative 1900. The BS-side of the relay 1910implements the WTRU protocol stack up to the PHY level. The PHYprocessing at the RS may be performed in alternative ways. A firstalternative, denoted amplify and forward (AF) relay, involves simpleamplification at the RF level and forwarding. A second alternative isdemodulate-remodulate-and-forward. In this alternative noise may beremoved or suppressed but only in those cases when the link to the relayis of very high fidelity (i.e. a channel code is effectively notnecessary). A third alternative is decode-reencode-and-forward (DF)relay. Here there is further processing of the signal such that theerrors are corrected at the RS.

During the relaying process, the radio signal characteristics on theBS-RS link and the RS-WTRU link need not be the same. In a first option,the RS-WTRU link may use different frequencies or codes compared to theBS-RS link. In a second option, the modulations on the RS-WTRU link maybe different from the modulation on the BS-RS link. In a third option,the error protection (i.e. detecting and/or correcting) codes may bedifferent on the BS-RS and RS-WTRU links.

Another type of relaying technique may be called compress and forward(CF) relay. This technique requires that there is an alternate signalpath between the BS and the WTRU, so that, for example in the downlink,the compressed signal sent from the relay can assist the direct signalfrom the BS. This configuration is addressed below in connection withdiversity mode.

FIG. 20 shows a second alternative for implementing the WTRU protocolstack 2000. In this alternative, the BS-side of the relay 2010implements the WTRU protocol stack up to the MAC level. This schemeprovides flexibility in the allocation of resources on the BS-RS andRS-WTRU links, due to the incorporation of the MAC protocol in the RS.It also allows for HARQ schemes separately for retransmission ofincorrectly received radio blocks between the BS-RS and RS-WTRU.

This example approach is highly transparent from a network/BS point ofview, as well as the WTRU point of view. Impacts include a modificationin the RLC protocol and an awareness on the part of the BS scheduler ofthe relay's DRX intervals.

The user-plane protocol stack in the relay (and therefore the complexityof the relay) is minimized. Beyond the necessary PHY-layer capabilities,the relay is required to maintain a “mirror” of the MAC for the WTRU (toemulate the WTRU in communicating with the BS) and a “mirror” of the MACfor the BS (to emulate the BS in communicating with the WTRU).

FIG. 21 show a third alternative for implementing the WTRU protocolstack 2100. In this alternative the BS-side of the relay 2110 implementsthe WTRU protocol stack up to the RLC level. This example allows forvarious radio link control (RLC) functions, such as link adaptation andretransmissions between the RS and the BS/WTRU.

The RLC protocol may be operating in any of acknowledged mode,unacknowledged mode, transparent mode or persistent mode. Furthermore,the modes of the RLC protocol between the BS and the RS and the RS andthe WTRU may be different.

The transfer of data between the BS and the WTRU via the RS(s) is nowdescribed by considering the transport of a single IP block. FIGS. 21-23are sequence diagrams showing various alternatives.

A first alternative is a PHY-level relay station. FIG. 22 illustratesthe sequence of events 2200 involved in transferring an IP packet fromthe BS 2210 to the WTRU 2220 via the RS 2230. The RS 2220 is assumed tobe a simple PHY-level variant of the amplify-and-forward ordecode-and-forward type.

The PHY processing at the RS 2220 may be performed in alternative ways.A first way, denoted as amplify-and-forward (AF) relay involves simpleamplification at the RF level and forwarding. A second way isdemodulate-remodulate-and-forward. In this way, noise may be removed orsuppressed but only in those cases when the link to the relay is of veryhigh fidelity (i.e. a channel code is effectively not necessary). Athird way is decode-reencode-and-forward (DF) relay. In this example,there is further processing of the signal such that the errors arecorrected at the RS 2230. A fourth way is denoted an adaptive scheme. Inthis scheme, the relay can switch between any of the above three schemesadaptively. For example, if the relay successfully decodes the message,it applies the decode-and-forward scheme. Otherwise, it may apply any ofthe remaining schemes, for example, the AF scheme. Another example isthat if the relay is heavily loaded, it may apply the AF scheme as itconsumes less resources. Otherwise, it may apply any of the otherschemes, such as the DF scheme.

During the relaying process, the radio signal characteristics on theBS-RS link and the RS-WTRU link need not be the same. In a first optionthe RS-WTRU link may use different frequencies or codes compared to theBS-RS link. In a second option the modulations on the RS-WTRU link maybe different from the modulation on the BS-RS link. In a third optionthe error protection (i.e. detecting and/or correcting) codes may bedifferent on the BS-RS and RS-WTRU links.

A second alternative for data transfer involves a MAC-level relaystation. In this example, the BS treats the relay as the WTRU andschedules to it accordingly, however this “relay-WTRU” has DRX cyclesthat the BS is aware of.

An ACK from the relay-WTRU is treated as a MAC-level (HARQ) ACK by theBS. The relay-WTRU then acts as a BS for the WTRUs it communicates with.One specific problem is what to do if the relay has ACK'ed a packet tothe BS, but is unsuccessful in delivering it to the WTRU.

One example of a general strategy that addresses this problem is asfollows. The relay (acting as a BS) attempts to deliver the packet a fewmore times, as per a MAC (HARQ) protocol. At some point, though, therelay has to give up. The relay cannot perform HARQ-NACK anymore sinceit has already ACK'ed (pretending to be the WTRU). So the NACK, ifneeded, has to take place at the RLC level. The WTRUs RLC is likely todo this already, provided that the RLC is running in an acknowledgedmode. Addressing the problem of the impossible MAC NACK cannot beeffectively addressed by delaying the MAC-level ACK. If such a delayoccurs, in principle, the relay could ask the BS for a re-transmission.Such a re-transmission however, would contain nothing but the data whichthe relay already possesses, therefore there is no reason to ask for it.In this example, the relay could just delay its HARQ time-out.

If the RLC is in an unacknowledged mode the packet is considered lost,which is acceptable (by definition of the unacknowledged mode). However,the network's RLC may need to be modified such that it is aware that aMAC-layer ACK is no longer an indication that a packet has indeed beendelivered.

FIG. 23 is a diagram of an example illustrating the sequence of events2300 involved for a MAC-level RS. FIG. 23 shown does not address theproblem of potential buffer overflows at the relay's MAC. Specifically,because the BS 2310 does not know when the relay has successfully (orun-successfully) forwarded the data to the WTRU 2320, it may attempt topush more data than the RS's buffer can handle. To prevent this, one ormore of the following options may be used. These options are notexclusive and may be combined.

In a first embodiment, the BS 2310 autonomously maintains an estimate asto the state of the relay buffer by knowing the maximal number ofattempts the RS 2330 makes to transmit data and how long it takes. Itcan also take into account the average time to deliver packets from theRS 2330 to the WTRU 2320, which can be periodically updated by therelay.

In a second embodiment, the RS 2330 periodically communicates bufferoccupancy status to the BS 2310. This may be performed by introducing anew MAC-level feedback signaling. This feedback signaling may becombined with other RS MAC-level feedback into a relay feedback channel(RFCH).

In a third embodiment, the RS 2330 incorporates buffer occupancy statusinto the feedback (together with, e.g. channel state information) thatthe BS 2310 uses to schedule its transmissions. There are a number ofoptions to do so. Frequently, standards provide a method for the WTRU2320 to report its ability to receive data via channel qualityindicators (CQIs). Such information may also be provided by the RS 2330to the BS 2310. By artificially reducing CQI, the RS 2330 may reduce theamount of data it receives, although in this case the BS 2310 neveractually knows whether the reduction is due to buffer occupancy issuesor channel conditions.

In a fourth embodiment, if its buffer is full, the RS 2330 responds witha special “buffer full” NACK whenever the BS 2310 attempts to push datato it. This is performed at the time of first BS-to-relay attempt andprevents the BS 2310 from re-transmitting until either a specifiedback-off time period has elapsed, or the relay issues a special datarequest communication to the BS 2310, or a free buffer is reportedabove.

A fifth embodiment, similar to the fourth, is to have a special ACK thatindicates that the buffer is almost full. In this example, the RS 2330will accept this packet, but not the next one. The BS 2310 behavior isthen similar to above—it can back off and/or wait for the RS 2330 toreport that its buffer is OK.

A sixth embodiment introduces a delayed secondary ACK/NACK from the RS2330 to the BS 2310 that may report to the BS 2310 when a delivery ofparticular packet to the WTRU 2320 succeeded or was abandoned. Thisallows the BS 2310 to maintain the state of the relay buffer andschedule accordingly.

A third alternative for data transfer involves an RLC-level RS. FIGS. 24a and 24 b illustrate the sequences of events 2400 involved in thisalternative. As shown in FIGS. 24 a and 24 b, the RS receives (and ifnecessary, acknowledges) the data packet before forwarding it to theWTRU. The BS considers this delivered and the RS is then responsible forthe delivery to the WTRU. No fallback mechanism is provided, nor isneeded. However, because of the RLC-level operation, the delayassociated with this process may be significantly larger than the delayassociated with MAC-level relaying.

RS-WTRU Channel

To understand how WTRU assignment to relays is performed in two-hopmode, the use of a dedicated RS-WTRU channel is now considered. Again,assume a single cell (i.e. single BS 2410), M RSs 2420 and multipleWTRUs 2430. The RSs 2420 are designed to improve the link qualitybetween the BS 2410 and the users (WTRUs 2430). The WTRUs 2430 aredivided into two categories. One category includes those WTRUs 2430 thatare connected to the BS 2410 and can communicate with the BS 2410without any assistance from a RS 2420. The other category includes thoseWTRUs 2430 that are disadvantaged in their connectivity to the BS 2410and require the assistance of the RS 2420. Furthermore, it is assumedthat each WTRU 2430 is associated with only a single RS 2420 at anygiven time. Since there are M RSs 2420, we can now define (M+1) WTRUgroups {G_(m), m=0, 1 . . . M}, as shown in FIG. 25.

WTRUs 2510 belonging to group Go communicate directly with the BS 2520,whereas the WTRUs 2530, 2535 belonging to group G_(m) communicate withthe BS 2520 via RS m 2540, 2550. For example, in the downlink, thepackets sent to the WTRUs in group 0 2560 follow the regular directtransmission, which is from the BS 2520 to the WTRUs 2510. Similarly,the packets sent to the WTRUs 2530, 2535 in groups 1, . . . , M need togo through a two-hop route, which is from the BS 2520 to the RS 2540,2550 and the RS 2540, 2550 to the WTRU 2530, 2535.

Which group each WTRU is in may be determined by different alternativecriteria. According to a first criterion, WTRUs in cell center belong toGroup 0 2560, while the WTRUs at the cell edge belong to Group 1, . . .M 2570, 2580. The cell edge/cell center differentiation may be madeusing one of the following techniques (or other methods): at the basestation, using round-trip time delay to/from the WTRU as evaluatedduring connection setup or while the connection is on-going. Such adelay may be evaluated because the WTRU in any system is required tosynchronize its transmission to a predefined BS signal in a predefinedway. The BS may then measure how out-of-sync the signal received fromthe WTRU is, such that the delay must be due to the round-trip delay. Ifthe BS has issued timing adjustment commands to the WTRU, these aretaken into account as well. At the WTRU, using estimated path loss,which may be computed by taking the difference between the received BSpower on some reference channel for which the transmitted power issignaled (most systems include at least one such channel). The celledge/cell center differentiation may also be made using an auxiliarylocation estimation device, such as GPS.

According to a second criterion the RS 2540, 2550 may monitor thecommunication between the WTRU 2530, 2535 and the BS 2520 as well as thechannel metrics they report to each other. The RS 2540, 2550 comparesthese metrics to its own observations of the relative channels anddetermines which WTRUs 2530, 2535 would benefit from communicatingindirectly through itself. The RS 2540, 2550 then manages the indirectconnection setup.

According to a third criterion, the RS 2540, 2550 periodically sends outa beacon signal which permits the WTRUs 2530, 2535 to determine whetherthey would benefit from using the relay and communicates thisinformation to the BS 2520. The WTRUs may send a response to the beaconssignals.

According to a fourth criterion, the WTRU 2530, 2535 periodicallyupdates its location (or other CELL_DCH and CELL_FACH signaling areused). The RS 2540, 2550 and BS 2520 may determine which group the WTRU2530, 2535 belongs to after they exchange their respective informationon WTRU's location or signaling.

Another technical issue is how handovers are executed as a WTRU comesinto the coverage region of a RS, moves out of the RS coverage regioninto the coverage region of another RS, or to the coverage region of theBS. Due to the WTRU's mobility, there is a need to enable changing theWTRU group dynamically (a sort of intra-cell inter-relay handover).Possible signaling strategies for enabling this are described in theexamples below.

Pooled Relay Transmissions

In some instances it may be beneficial for the BS to treat the RS 2540,2550 as a super-WTRU by pooling transmissions for all WTRUs associatedwith the RS. In this case, both the network and the WTRUs must be awarethat there is a RS (thus, it is not transparent)—and which WTRUs (ormore specifically, which RNTI's—there may be several per WTRU and alsocommon/shared ones) the relay handles. The network/BS pools these into asingle transmission, thus utilizing air interface more efficiently andlets the relay break them up.

To enable a pooled-relay transmission, additional and/or modifiedfunctionalities are required compared to a standard BS MAC. Theseinclude an RNTI pool manager, a MAC buffer manager, and modifiedscheduler. An RNTI pool manager entity maintains the RNTI groups andassociations between the actual RNTIs and the group RNTI allocated forthe purposes of communication with the relay. The MAC SDU arrive at theMAC associated with each individual data stream (each RNTI), howeverthese need to be multiplexed into the common stream by a MAC buffermanager.

To affect the best possible operation, it may be necessary to provide ascheduling preference to the grouped transmission to the relay. Theamount of preference is likely to depend on how many individual datastreams are grouped into the relay. A modified scheduler needs to beable to take all of this information into account. Because there is no1-1 correspondence between the BS-RS physical resources (shared) andRS-WTRU physical resources (dedicated), PHY-level cooperation is notfeasible in this case.

On the other hand, RLC-level resources are likely to remain dedicatedeven if they are mapped to a shared physical resource (witness, e.g.HSPA in WCDMA). Therefore, RLC-level cooperation is also not likely tochange from the previous case, provided that the MAC operation is welldefined.

FIG. 26 is an example diagram of a MAC-relay sublayer of the MAC 2600situated between the RS 2610 and the BS 2620. This layer handlestransmissions of the data on the grouped RNTI. The HARQ responses can behandled in alternative ways. In a first alternative a single ACK/NACK isgenerated by MAC-relay 2630 at the RS 2610. This is interpreted as anACK/NACK for all data included in the HARQ TTI and passed by a MAC-relay2640 to a MAC-WTRU 2650 in the BS/network 2620. In a second alternative,a separate ACK/NACK is generated for each data packet. This is alsopassed by a MAC-relay 2640 to a MAC-WTRU 2650 in the BS/network 2620. Inboth cases the MAC-WTRU-mirror 2660 in the relay is effectivelytransparent, it performs virtually no tasks.

On the transmit side, the RS 2610 now includes a more complex MAC entity2670 which may perform a number of tasks. It may schedule transmissionsfor RSs within a group. It may continue to schedule transmission forrelays between groups as before. It may maintain HARQ with the WTRU 2680(i.e. ACK/NACK exchange). The above protocol stack architecture can beextended to multi-hop relaying case. The data transfer operation in thisexample is the same as in the case of matched BS-RS and RS-WTRUresources. This was considered above.

The use of shared RS-WTRU channels is now described. If the RS-WTRUchannel is shared then the situation is similar to either the dedicatedBS-RS or dedicated RS-WTRU resource case (if the sharing is the sameacross both hops). Alternatively, scenarios where the sharing strategiesare not the same would result in operation similar to the example wherethe BS-RS link is shared and the RS-WTRU link is dedicated.

User plane system operations in diversity mode are disclosed in thefollowing. Considered first is dedicated BS-RS channels. In this mode,all WTRUs may dynamically switch its connection with the relay or theBS. The WTRUs may be also linked to the BS and the RS simultaneously,such that cooperative transmitter diversity is obtained. Since the WTRUsin such a mode have more flexibility and the connections between the BSand the WTRU and the BS and the RS are more dynamic, the schedulingalgorithm and system set up becomes more complicated. In particular, itis no longer possible for the RS to simply mirror the BS for the WTRUand the WTRU for the BS. The WTRU and the BS must be directly aware ofeach other as well as of the RS.

As in the case of a two-hop operation, the RS protocol stack must beconsidered. The system operation will be defined based on the RSprotocol stack, which can terminate at the PHY, the MAC or the RLClayer.

FIG. 27 is diagram of an example protocol architecture for a PHY-levelRS 2700. The dotted line indicates a direct connection between the BS2710 and the WTRU 2720. The PHY-relay operation only depends on thetransport format of PHY messages and L1 control (e.g. TPC). The protocollayers at the MAC and higher levels are unchanged at the BS and theWTRU.

This example represents a RS 2730. Such a RS assists in the datatransmission, but is not able to make any decision itself. The PHYprocessing at the RS 2730 may be performed in alternative ways.

A first alternative, denoted amplify & forward (AF) relay involvessimple amplification at the RF level and forwarding. A secondalternative is demodulate-remodulate and forward. In this alternativenoise may be removed or suppressed but only in those cases when the linkto the relay is of very high fidelity (i.e. a channel code iseffectively not necessary). A third alternative isdecode-reencode-and-forward (DF) relay. Here there is further processingof the signal such that the errors are corrected at the RS. A fourthalternative is compress-and-forward. Such a method may be used when thelink to the relay is poor and data cannot be demodulated even using achannel code. However, partial information about the data may still beobtained through the design of appropriate codes. This partial (i.e.“compressed”) information is forwarded by the relay to the destination.

A fifth alternative is Adaptive scheme. In this scheme, the relay canswitch between any of the above four schemes adaptively. For example, ifthe relay successfully decodes the message, it applies thedecode-and-forward scheme. Otherwise, it may apply any of the remainingschemes, for example, the amplify-and-forward scheme. Another example isthat if relay is heavily loaded, it may apply the amplify-and-forwardscheme as it consumes less resources. Otherwise, it may apply any of theother schemes, such as the decode-and-forward scheme.

During the relaying process, the radio signal characteristics on theBS-RS link and the RS-WTRU link need not be the same. In a first option,the RS-WTRU link may use different frequencies or codes compared to theBS-RS link. In a second option, the modulations on the RS-WTRU link maybe different from the modulation on the BS-RS link. Finally, in a thirdoption, the error protection (i.e. detecting and/or correcting) codesmay be different on the BS-RS and RS-WTRU links.

FIG. 28 is a diagram of an example protocol architecture for a MAC-levelRS 2800. The dotted line indicates the direct connection between the BS2810 and the WTRU 2820. As in the two hop-case, the introduction of theMAC 2830 in the RS 2840 allows the RS 2840 to participate in the HARQoperation of the system, potentially handle the retransmissions, etc.This flexibility allows for tremendous potential in improving theoverall system operation, as will be described when system operation isdiscussed. For now, it is noted that the usage of the RS MAC 2830 as amirror is no longer possible.

FIG. 29 is a diagram of an example protocol architecture for a RLC-levelRS 2900. The dotted line indicates the direct connection between the BS2910 and the WTRU 2920.

Now that the various physical architectures, protocol architectures aswell as radio channels have been delineated, the transfer of databetween the BS and the WTRU via the RS(s) may be described. Thetransport of a single IP block will be considered, as above.

FIG. 30 is a diagram of the sequence of events 3000 involved intransferring an IP packet from the BS to the WTRU via the RS. The RS3010 is assumed to be a simple PHY-level using any of the techniquesdescribed above. The approach shown applies to the decode-and-forwardand demodulate-and-forward approaches. In the case ofcompress-and-forward and amplify-and-forward, the RS 3010 may continueto receive successive refinements of the packet and transmit these(alternating receiver and transmit operations) until the transmissionsis complete (i.e. until the BS 3020 stops sending data).

Scheduling options include the following. In an example where the RS3010 must decode or demodulate a signal before any transmissions (i.e.excellent assurance of data before any transmission), it may bescheduled by the BS 3120 to transmit in the same TTIs as the BS 3020 orin different TTIs. In either case, since the WTRU 3030 ceases receptionof this particular packet, it needs to know when to stop transmission.This may be done in one of the following alternative ways.

In a first alternative, the actual transmissions are scheduled directlyby the BS 3020 using a side control channel (which the RS 3010 mustreceive). In a second alternative, the RS 3010 continues to monitor forre-transmissions, even if it no longer needs them. This carries minimalcontrol overhead, but since the RS 3010 is assumed to be half-duplex, itprevents operation where the BS 3020 and the RS 3010 send in the sameTTI using the same RRU. In a third alternative, a PHY-layer controlsignal is used whereby the BS 3020 notifies the RSs 3010 which packetsit is still actively transmitting or when it has stopped transmittingpackets. This alternative carries some signaling overhead, but retainsfull flexibility in scheduling the RS's 3010 RRUs (e.g. to overlap withthe BS 3020).

In the case when the RS 3010 is capable of switching between transmitand receive operations dynamically, the options above are still viable.The difference is that now the RS 3010 has several options as to when tostart transmitting.

As a first option, if it waits until it has successfully decodedinformation, the situation is the same as described for the two-hopoperation, therefore only cases when relay starts transmitting beforefull information is available are considered further. As a secondoption, it can transmit something (what specifically depends on theprotocol) after each BS transmission, until it successfully decodesdata, at which point it continues transmit only (no receive) until ithas stop, as in the decode-and-forward case above. As a third option,alternatively, the relay waits until its accumulated information isabove some quality threshold or it has received at least some minimalnumber of transmissions from the BS.

The MAC level cooperation will now be considered. Given a relay with aPHY+MAC layers, two fundamentally different approaches to cooperativetechnique may be considered that have to do with how the relay iscontrolled by the BS. In one case, the BS is not aware of the detailedrelay operation and does not apply fine control, and in the other itdoes. Because the BS transmission is always available, the relayingprotocol maybe based either on the decode-and-forward (DF) orcompress-and-forward (CF) approach.

In the case when the BS is not aware of the detailed relay operation,the data transfer operates as shown in FIG. 31. The BS 3110 continuessending packets to the WTRUs 3120 until an ACK is received from the WTRU3120 or the maximum number of transmission times is exceeded. A RS 3130may join the transmission when it is ready (which depends on whether itis doing CF or DF) and if the WTRU 3120 sends a NACK to the BS 3110.Therefore, in this mode, cooperative transmitter diversity is activatedno earlier then for the first re-transmission, and only if the channelto the RS 3130 is truly better, then to the WTRU 3120 (thus allowing theWTRU 3120 to receive the data first).

A key factor in this case is the definition of a RS 3130 being “READY totransmit.” If DF is used, this means that the relay has successfullydecoded the BS transmission. If CF is used, this means that the relayhas accumulated sufficient amount of information about BS transmissionto pass some pre-defined threshold.

Two approaches to using a relay without detailed BS control may bedefined. In a first approach, called smart relay, every frame includescertain dispersed RRU which are reserved for RSs 3130. Beyond thispre-allocation, the BS 3110 does not know what the RSs 3130 will do withthese. A RS 3130 determines which WTRUs are in its “assist set”, i.e.whether the BS 3110 can process the CQI information to determine whichWTRUs 3120 are in assist set and signal that information to the RS 3130.

FIG. 32 is a diagram illustrating example signal flows 3200 for a smartrelay and a slave relay. Referring to FIG. 32, the smart RS 3210monitors the BS transmissions to these WTRUs 3220. It also monitors theWTRU feedback 3220 (ACK/NACK). Once the RS 3210 is READY 3215 and theWTRU 3230 sends a NACK 3235 (which goes to the BS 3240, but the RS 3210sees it) it will schedule a re-transmission 3250 to the WTRU 3230. Theretransmissions are scheduled slots (as well as other physicalresources, such as channelization codes and/or sub-carriers) that areeither a) previously assigned by the BS to the RS for transmissions tothe WTRU of b) chosen from a set of slots that the relay is allowed touse to schedule to any of the WTRUs that are associated with it. The BS3240 will in general be aware of the rules that the RS 3210 uses—thus itcan estimate which WTRUs 3230 should be in which RS's 3210 “assist set,”and make its retransmission decision accordingly (i.e. schedulere-transmissions less frequently hoping the relay will pick up the load.Alternately, the RS 3210 can explicitly signal to the BS 3240 the WTRUs3230 it is currently assisting or plan to assist in future). A WTRU 3230that is being assisted is signaled by the RS 3210 with its index. Inthis way, the WTRU 3230 may switch its receiver operation to supportcooperative transmission from this WTRU 3230.

In a second approach, called slave relay 3260, only after BS 3240 getsNACK 3270 from WTRUs 3230 in assist set (similar to Approach 1, BS canbase on CQI information to determine which WTRUs are in assist set), theBS 3240 signals 3245 to RS 3210. When READY 3280, RS 3210 beginstransmissions 3290 to the WTRU 3230. It continues until signaled tostop. With such an approach, RS 3210 does not need to detect/share theACK/NACK information sent from the WTRU 3230 to the BS 3240.

Now considered is an example MAC-level cooperation with fine controlfrom the BSs. In this mode of operation, a two-level HARQ is required(between WTRU and BS and between relay and BS). Using a DF or a CFcommunication scheme, the relay informs the BS when it is ready using aspecial relay ACK (RAC), which is associated with a particular HARQprocess. At this point, the BS uses direct signaling to tell the relaywhich packets should be transmitted in which RRUs. The scheduling may beperformed for each transmission or in bulk (i.e. “until success ortime-out”). If it is performed in bulk, the relay may be instructed tostop by the BS, or it may be told to monitor ACK/NACK from WTRU and stopwhen an ACK is detected. The BS may use any of the following methods toschedule relay transmissions. The first method maximizes MIMOeffectiveness by scheduling transmissions in the same RRUs. The secondmethod maximizes time diversity/minimizes interference by schedulingtransmissions in different RRUs.

Now considered are WTRU-controlled adaptive NACK transmissions. In thisscheme, the WTRU keeps track of the quality of the WTRU-BS and WTRU-RSchannels and selectively sends NACK transmissions to either RS or BS orboth.

The selection criterion may be, for example, based on the channelquality of the respective channels. That is, the WTRU may select to sendthe NACK to the network node that is estimated to have a higherprobability of successful retransmission.

The selective transmission may be performed at the PHY level or higherlevels. At the PHY level, directive antennas at the WTRU may be used toselectively send the NACK transmission to the RS or the BS. At higherlevels, the NACK message may contain an identifier, which identifies themessage as being meant for the RS or the BS. Non-selective transmissionare achieved with omni-antennas (or broad-beamed antennas) at the PHYlevel and as broadcast messages at higher levels.

Now considered are control plane system operations. The keycontrol-plane operation which must be addressed in the context of a ModeA relay configuration is that of mobility management, i.e. themanagement of the mobility of a WTRU between different relay groups,including group 0 (no relay in use).

As discussed above, the measurement associated with allocating a WTRU toa particular relay group may be performed in different places in thecell, such as at the WTRU, at the relay or at the BS. A combination ofthese may be used.

These measurements are provided to the BS/networks irrespective of wherethe measurements are performed. Based on these measurements, the BS(likely the BS and not the network) allocates the WTRU to a specificrelay group and forwards the appropriate command to the relays involved(the originating and receiving relay) and the WTRU. In fact, from acontrol point of view the BS acts as an RNC in the modern WCDMA systemwhile relays act as BSs. Such an operation necessitates the followingchanges to the control-plane access stratum protocol stack.

FIG. 33 a is a diagram of an example protocol architecture 3300 wherethe BS and the relay contain a Layer 2 contour plane entity (currentlynot present in many systems, such as WCDMA as RRC is Layer 3), which weshall call relay-RRC 3310. This entity manages the mobility of WTRUsbetween relay groups. FIG. 33 b is a diagram of an alternate exampleprotocol architecture 3300 where the BS and the relay contain arelay-RRC 3310.

As the WTRU is generally not aware of which group it belongs to, such anentity may not be required at the WTRU. When the WTRU performs themeasurements which support inter-relay mobility, the existing RRCoperation may need to be modified to provide these to the network.Alternatively, a Layer 2 RRC entity may be defined to report these tothe BS.

Turning our attention to the specific coding process in the presence ofa relay, consider FIG. 34 which is a diagram of an example cooperativeheader 3400. Referring to FIG. 34, the cooperative header 3410 includes,but not necessarily limited to 2 bits 3420 indicating M0 or M1 or M2(each representing a different relay method), k bits indicating thespecific embodiment in M1 or M2 or {tilde over (k)} bits indicatingother cooperative scheme specific details. These “information bits” maybe compactly coded to generate the cooperative header. For example, the2 bits indicating M0/M1/M2 and K bits may be coded together therebyeliminating the unused code point in the 2 bit field (because 2 bits=4code points, where are there are only 3 modes to be coded) and possiblyreducing the header size by 1 or more bits (instead of 2+k+{tilde over(k)} bits).

The Data Packet is further processed for radio transmission (e.g., errorcorrection/detection codes are applied, modulated etc). The transmittedsignal s(t) is carried on a “common” radio channel, which is heard bythe WTRU, RS1 and RS2, where RS1 and RS2 are associated with the WTRUand “common” refers to the WTRU, RS1, RS2, not necessarily common toother WTRUs or RSs in the system.

Since the channel qualities are different on the BS→RS1, BS→RS2 andBS→WTRU channels, channel coding, modulation similar “transmissionrelated parameters” must be selected such that all 3 entities, namelythe WTRU, RS1 and RS2, are able to correctly decode the header. This maybe performed using several techniques, some of which are discussedbelow.

FIG. 35 is a diagram of an example technique 3500 that may separatechannel coding for the header and the payload, for example a FEC1 3510for the header and a FEC2 3520 for the payload, where FEC1 3510 isstronger than FEC2 3520. An example overall flow diagram (shown for DLsimilar ones apply for UL) is shown in FIG. 36. This technique may usechannel coding FECI 3510 for both header and payload, where FEC1 3510 ischosen to be strong enough for header to be reliably received by WTRU,RS1 and RS2. Robust coding of mode bits, for example 2 mode bits, may beseparately coded and modulated and symbols are placed at fixed locations3610 (which are known to WTRU, RS1 and RS2), so that WTRU, RS1, RS2 maydemodulate and decode Mode-Symbols without having to decode the entireheader or/and payload. Other similar schemes may be used based onvariations of these examples.

The data packet is received by the WTRU, the RS1 and the RS2 3620. Eachof them detects, demodulates and decodes the mode bits 3630. Dependingon the value of the mode-bits, the nodes (WTRU, RS1, RS2) behaveaccordingly. That is, if Mode 0 is indicated 3635, RS1 and RS2 will stopfurther processing of the data block while the WTRU decodes the datapacket 3640. The WTRU will continue processing if Mode 1 is indicated3645, the RS2 and the WTRU will stop further processing, whereas RS1will continue 3650. If M1 or M2 are chosen, then Phase 2 will commence3655 after data packet has been correctly received by RS1 or RS2 3660.Upon receiving the decoded data packet, the WTRU sends an ACK/NACK tothe BS or RS 3670.

The structure of the data packet sent in Phase 2 of M1 or M2 need notcontain the cooperative header. This saves some bits from beingunnecessarily transmitted, leading to reduced interference, increasedthroughput, etc.

DL & UL Coordination

So far, solutions for DL & UL have been described separately. Nextdescribed are ways to efficiently coordinate them.

The basic idea is that a DL data or control packet contains informationabout the cooperative scheme to be used for UL transmissions. Thefollowing is an example of “piggy-backing”. FIG. 37 shows a downlinkData Packet having a header and a payload 3700. The header 3710comprises a DL-coop header 3720, and an UL-coop header 3730 thatcontains details of the cooperative scheme to be used in the “next”UL-cooperative transmission. As a variation, a time period during whichthe UL-coop scheme should be used is specified. The specification of the“time-period” may be in terms of “absolute” time (e.g. time slotnumbers) or in “logical time” (e.g. Temporary Block Flow identitiesetc.). The UL-coop header 3730 may also contain the address of WTRU, incase multiple WTRUs are served.

Regarding the availability of channel state information, please notethat in the above descriptions, it is assumed that the BS hadinformation about the five channel states 1-5 shown in FIG. 38, or asubset thereof. This information may be acquired in a variety of ways.For example, it may be fed back from the WTRU 3810, RS1 3820, or RS23830 on a periodic basis or upon being polled by the BS 3840, RS1 3820or RS2 3830. The period of feedback reporting may be adjusteddynamically or it may be fixed at the beginning of the communicationsuch that the overhead and latency incurred is acceptable. The BS 3840or RS 3820,3830 may use interpolation or prediction methods to estimatethe channel state values in between feedback reports. In TDD systems,the channel states may be estimated by the BS 3840 on the assumption ofreciprocity of the DL and UL channels.

FIG. 39 shows a transmission header 3900 comprising a “legacy” header3910 appended by 1 bit, called “Coop. Header indicator Bit” 3920. Onevalue of this bit denotes the presence of a cooperative header 3930. Theother (binary) value of this bit denotes the absence of the cooperativeheader 3930.

In this example, the cooperative header 3930 denotes only M1 or M2 modes(i.e. all modes involving a relay), excluding the mode M0 (i.e. thedirect BS-WTRU communication). So the absence of the cooperative header3930 denotes mode M0. This will reduce the average header size, whenobserved over many transmissions.

A variation to using a “bit” to indicate the presence or absence ofcooperative header 3930, any unused “code-point” from the legacy header3910 may be used. That is, an unused bit pattern of the legacy header isused. A variation is to use rate compatible punctured convolution (RCPC)code, which has the capability to protect different parts of the datapacket to different levels.

In one example, the BS and RS transmit the same channel coded data, withdistributed beam-forming (BF), so that the WTRU receives the coherentlycombined signal with improved SINR. This method requires someinformation feedback from the WTRU to the BS and RS (e.g. channel stateinformation or beam-forming weights), and may be viewed as a variant ofthe ‘closed loop transmit diversity’ scheme. In another example, the BSand RS transmit different parts of the coded bit stream, which arereceived by the WTRU and separated by successive interferencecancellation techniques. Subsequently, the two demodulated bit streamsare combined at the channel decoding level. Coupling this with thepartial data received in Phase 1, the WTRU completes the reconstructionof the data originally transmitted by the BS. These examples arereferred to as distributed-BF and distributed-MIMO collaborativeschemes.

Protocol 1 Operations

Protocol 1—Scheme 1 Downlink is shown in FIG. 40. Referring to FIG. 40,the BS may send data 4010 to the RS in a first TTI using an MCS suitablefor the BS-RS link, or one that considers the overall BS-RS, BS-WTRU,RS-WTRU links. The BS and RS may send data 4020 to the WTRU in asubsequent TTI using an MCS suitable for the RS-WTRU link, or one thatconsiders the overall BS-WTRU and RS-WTRU links. The WTRU receives asingle codeword (e.g. HARQ PDU) in a TTI, that is transmitted either bythe BS alone 4030, or jointly (e.g. using a distributed space-time code)by both the BS and the RS 4040, (or as a third possibility by the RSalone (not shown)).

This may be extended/generalized to multiple codewords, e.g. if MIMOtransmission is used from the BS and/or the RS to the WTRU. The codewordtransmissions are described/indicated to the WTRU via control channel(s)(i.e. TCC). The WTRU may send HARQ feedback (e.g. ACK 4040/NACK 4050) toindicate whether a codeword has been received successfully or not. Suchfeedback can be sent using the HCC channel(s). The RS may send HARQfeedback (e.g. ACK/NACK) to the BS (not shown) to indicate whether acodeword transmitted by the BS has been received successfully or not bythe RS. Such feedback can be sent using the HCC channel(s).

If the HARQ feedback indicates that the RS has not successfully receivedthe codeword (i.e. NACK or DTX), the BS may re-transmit (not shown).Retransmitted packets will preferably have different IR version. If theBS receives an ACK from the WTRU, the BS moves on to transmit the nextmessage/packet. If the BS and/or RS do not receive an ACK from the WTRU,both the BS and RS will conduct retransmissions to the WTRU (e.g. usinga distributed space-time code), until the WTRU acknowledges (sends anACK) or until HARQ retransmissions are exhausted. Retransmitted packetsmay have a different IR version. The WTRU combines the received versions(e.g. HARQ combining) in order to improve the decoding of a given packetm. Common identifiers are employed by the BS and RS in order to enablethe WTRU to recognize which packets to combine. Such identifiers may bein the form of (using the same) HARQ process ID, pre-defined TTI's (e.g.At TTI # x+y, the RS will send the packet received from BS in TTI # x),or any other identification form. The uplink description is similar tothat of downlink but the BS and the WTRU roles are switched.

Protocol 1—Scheme 2 downlink is shown in FIG. 41, is generally similarto Scheme 1, with the following differences. A pair of TTI's is usedsuch that HARQ feedback is transmitted by the WTRU at the end of thelatter TTI 4110 (as opposed to transmitting HARQ feedback in each TTI).This can also be generalized/extended to a ‘bundle’ of 2 or more TTI'sinstead of a ‘pair’ of TTI's. The uplink description is similar to thatof downlink but with the BS and the WTRU roles switched.

Protocol 2 Operations

Protocol 2—Scheme 1 Downlink as shown in FIG. 42, describes a HARQscheme for protocol 2 which has a full-duplex relay 4200, i.e. the RS iscapable of simultaneous reception and transmission (e.g. on differentfrequencies). The BS may send data to the RS using an MCS suitable forthe BS-RS link. In TTIs when the RS is expected to be (or is) busytransmitting to the WTRU, the BS may send data to the WTRU using an MCSsuitable for the BS-WTRU link. In this example, the RS also receivessuch transmissions 4210 from the BS to the WTRU, because of itsfull-duplex nature. The RS may send data to the WTRU using an MCSsuitable for the RS-WTRU link. The WTRU receives up to two codewords(e.g. HARQ PDUs) in a TTI, one from the BS and one from the RS. This canbe extended/generalized to more than 2 codewords, e.g. if MIMOtransmission is used from BS and/or RS to WTRU, or if more than one RSis used. The codeword transmissions are described/indicated to the WTRUvia control channel(s) (i.e. TCC).

The WTRU may send HARQ feedback (e.g. ACK 4220/NACK 4230) to indicatewhether each of the two codewords has been received successfully or not.Such feedback can be sent using the HCC channel(s). The RS may send HARQfeedback (e.g. ACK/NACK) to the BS (not shown) to indicate whether acodeword transmitted by the BS has been received successfully or not bythe RS. Such feedback may be sent using the HCC channel(s). If the HARQfeedback indicates that the RS has not successfully received thecodeword (i.e. NACK or DTX), the BS may re-transmit (not shown).Retransmitted packets may have a different IR version. If the BSreceives an ACK from the WTRU, the BS moves on to transmit the nextmessage/packet. If the BS receives an ACK from the RS, the BS moves onto transmit the next message/packet. HARQ retransmissions may bedelegated to the RS. If the RS does not receive an ACK from the WTRU,the RS will conduct retransmissions to the WTRU, until the WTRUacknowledges (sends an ACK) or until HARQ retransmissions are exhausted(reach a limit). Retransmitted packets may have a different IR version.The WTRU combines the received versions (e.g. HARQ combining) in orderto improve the decoding of a given packet m. Common identifiers areemployed by the BS and RS in order to enable the WTRU to recognize whichpackets to combine. Such identifiers can be in the form of (using thesame) HARQ process ID, pre-defined TTI's (e.g. At TTI # x+y, the RS willsend the packet received from the BS in TTI # x), or any otheridentification form. Flow control signals may also be used from the RSto BS to stop new HARQ transmissions by the BS, when the RS isoverloaded with HARQ retransmissions to the WTRU.

The uplink is similar to that of downlink but the BS and the WTRU rolesare switched. The description is also similar, just replace BS by WTRU,and WTRU by BS as follows. This example has full-duplex relay, such thatthe RS is capable of simultaneous reception and transmission (e.g. ondifferent frequencies). The WTRU sends data to the RS (preferably usingan MCS suitable for the WTRU-RS link). In TTIs when the RS is expectedto be (or is) busy transmitting to the BS, the WTRU may send data to theBS (preferably using an MCS suitable for the WTRU-BS link). The RS mayalso receive such transmissions from the WTRU to the BS, because of itsfull-duplex nature. The RS sends data to the BS (preferably using an MCSsuitable for the RS-BS link). The BS receives up to two codewords (e.g.HARQ PDUs) in a TTI, one from WTRU and one from RS. [Note: This can beextended/generalized to more than 2 codewords, e.g. if MIMO transmissionis used from WTRU and/or RS to BS, or if more than one RS is used.]. Thecodeword transmissions are described/indicated via control channel(s)(i.e. TCC).

The BS may send HARQ feedback (e.g. ACK/NACK) to indicate whether eachof the two codewords has been received successfully or not. Suchfeedback can be sent using the HCC channel(s). The RS may send HARQfeedback (e.g. ACK/NACK) to the WTRU (not shown) to indicate whether acodeword transmitted by the WTRU has been received successfully or notby the RS. Such feedback can be sent using the HCC channel(s).

If the HARQ feedback indicates that the RS has not successfully receivedthe codeword (i.e. NACK or DTX), the WTRU may re-transmit [Note: this isnot shown in the Figure]. Retransmitted packets will preferably havedifferent IR version. If the WTRU receives an ACK from the BS, the WTRUmoves on to transmit the next message/packet. If the WTRU receives anACK from the RS, the WTRU moves on to transmit the next message/packet.HARQ retransmissions will be delegated to the RS. If the RS does notreceive an ACK from the BS, the RS will conduct (take care of)retransmissions to the BS, until the BS acknowledges (sends an ACK) oruntil HARQ retransmissions are exhausted (e.g. reach a predeterminedlimit). Retransmitted packets may have a different IR version. The BScombines the received versions (e.g. HARQ combining) in order to improvethe decoding of a given packet m. Common identifiers are employed by theWTRU and RS in order to enable the BS to recognize which packets tocombine. Such identifiers may be in the form of (using the same) HARQprocess ID, pre-defined TTI's (e.g. At TTI # x+y, the RS will send thepacket received from WTRU in TTI # x), or any other identification form.Flow control signals may also be used from RS to WTRU to stop new HARQtransmissions by the WTRU, when the RS is overloaded with HARQretransmissions to the BS.

Protocol 2—Scheme 2 Downlink is generally similar to Scheme 1 and isshown in FIG. 43, with the following differences. In TTIs when the RS isexpected to be (or is) busy transmitting or re-transmitting to the WTRU,the BS may conduct some HARQ retransmissions 4310 to the WTRU using anMCS suitable for the BS-WTRU link. Whether the BS takes care ofconducting retransmissions or not can be based on ACK/NACK feedbackstatus from the RS and/or WTRU, and/or RS load. The uplinkdrawing/figure and description is similar to that of downlink but the BSand the WTRU roles are switched.

Protocol 2—Scheme 3 downlink, as shown in FIG. 44 is generally similarto Scheme 2, with the following differences. First, a pair of TTI's 4410is used and the HARQ feedback is transmitted by the WTRU 4420 at the endof the latter TTI (as opposed to transmitting HARQ feedback in eachTTI). This may also be generalized/extended to a ‘bundle’ of 2 or moreTTI's instead of a ‘pair’ of TTI'S. Second, the uplink description issimilar to that of downlink but with the BS and the WTRU roles switched.

Protocol 2—Scheme 4 downlink, as shown in FIG. 45, is generally similarto Scheme 2, with the following differences. First, HARQ retransmissionsfor some packets will not be delegated from the BS to the RS 4510, butthe HARQ retransmissions for some other packets will be delegated fromthe BS to the RS 4520. Whether to delegate or not can be based onACK/NACK feedback status from the RS and/or WTRU, and/or RS load.Second, the uplink drawing/figure and description is similar to that ofdownlink but with switching/re-labeling BS as WTRU, and WTRU as BS.

Protocol 2—Scheme 5 Downlink, as shown in FIG. 46, is generally similarto Scheme 1, with the following differences. First, this scheme hashalf-duplex relay, such that the RS is capable of either reception ortransmission, but not both at the same time. Second, the HARQretransmissions for some packets will not be delegated from the BS tothe RS 4610, but the HARQ retransmissions for some other packets will bedelegated from the BS to the RS 4620. Whether to delegate or not may bebased on whether the RS has received the packet from the BS (i.e.whether the RS was receiving or transmitting, since it's half-duplex).Other factors such as ACK/NACK feedback status from the RS and/or WTRU,and/or RS load may also be considered. Third, the uplink description issimilar to that of downlink but with the BS and the WTRU roles switched.

Protocol 2—Scheme 6 downlink, as shown in FIG. 47, is generally similarto Scheme 5, with the following differences. A pair of TTI's 4710 isused and the HARQ feedback is transmitted by the WTRU at the end of thelatter TTI 4720 (as opposed to transmitting HARQ feedback in each TTI).This may also be generalized/extended to a ‘bundle’ of 2 or more TTI'sinstead of a ‘pair’ of TTI's. The uplink description is similar to thatof downlink but with the BS and the WTRU roles switched.

The physical channel denotes and differentiates the various ways inwhich physical resources are allocated among WTRUs, relay stations andbase stations. A physical channel, as used here, is a specific set ofresources associated with a specific terminal (i.e. a WTRU), set ofterminals, cells etc. More specifically, a physical channel in acellular system may be defined by a direction (uplink UL or downlinkDL), a carrier frequency, a cell or sector of the cellular system, andchannelization resources, as appropriate to a specific radio accesstechnology. Thus, in time division multiple access (TDMA), this is a setof time-slots, in code division multiple access (CDMA), this is a set ofcodes, in orthogonal frequency division multiple access (OFDMA) this isa set of sub-carriers, in time division duplex CDMA (TDD-CDMA) this is acombination of time-slot and code, and so forth.

Channelization resources are allocated as sets of radio resource units(RRUs). A radio resource unit is the smallest particular allocation ofresources in a specific radio access technology. For example, forwideband CDMA (WCDMA) HSDPA, 1 RRU=1 SF16 code*1 TTI. For long termevolution (LTE), 1 RRU=1 sub-carrier*1 TTI.

Generally speaking, physical channels assigned to terminals (WTRUs) maybe of, but not limited to, the following three types. First, a dedicatedphysical channel is allocated to a specific WTRU for its exclusive use.This allocation may be dynamic, that is, a shared pool of RRUs may beused, but each RRU is dedicated to a single WTRU. For example, WCDMAHSDPA as originally defined in Release 5 of the UMTS WCDMA standard is adedicated allocation. Even though the physical HSDPA channel (HS-PDSCH)is shared, each RRU therein is allocated in a dedicated fashion. Second,a shared channel is shared among a well-defined (static or dynamic) setof WTRUs. Third, a common channel is available to any terminal in thespecified cell.

The number of available RRUs depends on how RRUs are defined and howthey are received. These examples apply to all the approaches to RRUdefinition and reception described below.

In general it may be assumed that the RRU's are non-interfering ororthogonal (as, for example, time-slots with sufficient gap periods orsub-carriers of OFDM are). While this ensures the best performance foreach link, the overall system performance is limited by the availabilityof orthogonal RRUs.

An alternative to this is to allow some small amount of interferencebetween RRUs and ignore this in the receiver design. This is the casewith long-code CDMA with a RAKE receiver. This removes the RRUavailability as a factor limiting system performance, however suchsystems are typically limited by self-interference levels. Thus, while alarge number of RRUs is available in principle, very few of these canactually be used simultaneously. The actual RRU efficiency of suchsystems is often similar to those with orthogonal RRUs (and is oftensomewhat worse).

A theoretically-optimal approach is to permit some (limited andcontrolled) interference between RRUs and use a very powerful receiverto jointly receive all RRUs in a self-interference set. A partial stepin this direction was taken by WTDD TDSCDMA modes of 3GPP.

Control Channels

The following control channel architecture may be used in conjunctionwith both Protocol 2 and Protocol 1. Two types of control channels aredescribed herein. TCC's are Control channels that describe or provideinformation about the associated (data) transmissions. For example,describing when transmissions will occur, the MCS used, newtransmissions or retransmissions, IR version, etc. HCC's are Controlchannels that describe or provide information about the receptionstatus. For example, HARQ ACK/NACK feedback to indicate whether atransmission was received successfully (ACK), unsuccessfully (NACK) ornot received (DTX; i.e. no feedback is transmitted).

FIG. 48 shows control channels for the DL 4800. The WTRU 4810 monitors acontrol channel transmitted by the BS 4820 (referred to as TCC1 4830),that signals information regarding the transmissions from the BS 4820.The WTRU 4810 monitors a control channel transmitted by the RS 4840(referred to as TCC2 4850), that signals information regarding thetransmissions from the RS 4840. Alternatively, TCC2 4850 may betransmitted by the BS 4820 instead, but still signals informationregarding the transmissions from the RS 4840. TCC1 4830 and TCC2 4850may be combined in one control channel (i.e. a single TCC from BS).

The RS 4840 monitors a control channel transmitted by the BS 4820(referred to as TCC3 4860), that signals information regarding thetransmissions from the BS 4820. TCC1 4830 and TCC3 4860 may be the samecontrol channel (i.e. a single TCC from BS). The WTRU 4810 transmits aHARQ feedback control channel (referred to as HCC1 4870) to the BS 4820.The WTRU 4810 transmits a HARQ feedback control channel (referred to asHCC2 4880) to the RS 4840. The RS 4840 transmits a HARQ feedback controlchannel (referred to as HCC3 4890) to the BS 4820. HCC1 4870 and HCC24880 may be the same control channel (i.e. a single HCC from the WTRU).

FIG. 49 shows Variant A of the control channels for UL 4900. The WTRU4910 monitors a control channel transmitted by the BS 4920 (referred toas TCC1 4930), that signals information regarding the transmissions fromthe WTRU 4910 (i.e. it instructs the WTRU 4910 when and/or what totransmit to the BS 4920). The WTRU 4910 monitors a control channeltransmitted by the RS 4940 (referred to as TCC2 4950), that signalsinformation regarding the transmissions from the WTRU 4910 (i.e. itinstructs the WTRU 4910 when and/or what to transmit to the RS 4940).Alternatively, TCC2 4950 may be transmitted by the BS 4920 instead, oryet alternatively TCC1 4930 and TCC2 4950 may be the same controlchannel (e.g. a single TCC from the BS to the WTRU that instructs theWTRU when and/or what to transmit to either of or both RS and BS).

The RS 4940 monitors a control channel transmitted by the BS 4920(referred to as TCC3 4960), that signals information regarding thetransmissions from the RS 4940 (i.e. it instructs the RS 4940 whenand/or what to transmit to the BS 4920 and/or to the WTRU 4910). TCC14930 and TCC3 4960 may be the same control channel (i.e. a single TCCfrom the BS 4920) to the WTRU 4910 and/or the RS 4940 that instructs theWTRU 4910 and the RS 4940 when and/or what to transmit. The WTRU 4910receives a HARQ feedback control channel (referred to as HCC1 4970) fromthe BS 4920. The WTRU 4910 receives a HARQ feedback control channel(referred to as HCC2 4980) from the RS 4940. The RS 4940 receives a HARQfeedback control channel (referred to as HCC3 4990) from the BS 4920.HCC1 4970 and HCC3 4990 may be the same control channel (i.e. a singleHCC from BS 4920). The UL control channels (TTCx or HCCx) are notnecessarily the same as the DL control channels, although the same termsare used in the description.

FIG. 50 shows Variant B of the control channels for UL 5000. Variant Bdescribes a WTRU 5010 that transmits a control channel to the BS 5020(referred to as TCC1 5030), that signals information regarding thetransmissions from the WTRU 5010. The WTRU 5010 transmits a controlchannel to the RS 5040 (referred to as TCC2 5050), that signalsinformation regarding the transmissions from the WTRU 5010.Alternatively, TCC1 5030 and TCC2 5050 may be the same control channel(i.e. a single TCC from the WTRU 5010).

The RS 5040 transmits a control channel to the BS 5020 (referred to asTCC3 5060), that signals information regarding the transmissions fromthe RS 5040. The WTRU 5010 receives a HARQ feedback control channel(referred to as HCC1 5070) from the BS 5020. The WTRU 5010 receives aHARQ feedback control channel (referred to as HCC2 5080) from the RS5040. The RS 5040 receives a HARQ feedback control channel (referred toas HCC3 5090) from the BS 5020. HCC1 5070 and HCC3 5090 may be the samecontrol channel (i.e. a single HCC from BS 5020). The UL controlchannels (TTCx or HCCx) are not necessarily the same as the DL controlchannels, although the same terms were used in the description. Othervariants are also possible via combining some aspects from Variant Atogether with other aspects of Variant B.

Several example protocols are described to improve the downlinkperformance of cellular systems. These protocols are designed to buildupon existing cellular packet air interfaces such as high-speed packetaccess (HSPA) high-speed downlink packet access/high-speed uplink packetaccess (HSDPA/HSUPA) and long term evolution (LTE). While theseprotocols are disclosed in the context of HSPA, the protocols asdescribed, apply directly to other systems, such as LTE, WiMAX.

Cooperative Relays in HSUPA

The link between the relay and the WTRU may be classified as one-to-oneor one-to-many. In the one-to-one link, the relay is dedicated to asingle WTRU. In the one-to-many scenario, the relay is receiving datafrom multiple WTRUs. Similarly, the link between the relay and the BSmay be one-to-one or one-to-many. In the one-to-one scenario, the BS isreceiving data from a single relay and in the one-to-many scenario thebase station is receiving data from multiple relays. Finally, there isalso a direct link between the BS and the WTRU. This link might or mightnot be present. We could define an architecture where the WTRU cannotcommunicate directly with the BS, i.e., all communication goes throughthe relays. This however, would be a limiting architecture because therelay's objective is to help the communication between the WTRU and theBS, and so there will be cases where the relay is not needed, and directcommunication between the WTRU and the BS is advantageous. This link isalways defined as one-to-many. Finally, the WTRU may be in communicationwith both the relay and the BS at the same time.

Uplink communication WTRU Relay BS WTRU NA One-to-one or One-to-manyone-to-many Relay One-to-one or NA One-to-one or one-to-many one-to-manyBS One-to-many One-to-one or NA one-to-many

In order to generalize it here we will assume all links are one-to-many.The one-to-one case is the trivial case when there is a singledestination.

HSUPA Serving Grant Methodology

The HSUPA channel is the Enhanced Dedicated Physical Channel (EPDCH).The BS controls the allocation of the E-DPCH among all WTRUs and thiscontrolled scheduling is based on a set of rules on how the WTRU shallbehave with respect to specific signaling.

The BS sends a resource indication in the downlink called a “schedulinggrant” (SG). This SG indicates to the WTRU the maximum amount of uplinkresources it may use. When issuing scheduling grants, the BS may useQoS-related information provided by the SRNC and from the WTRU inScheduling Requests.

The scheduling grants have the following characteristics: schedulinggrants control the maximum allowed E-DPDCH/DPCCH power ratio, andscheduling grants can be sent once per TTI or slower. There are twotypes of grants.

The absolute grants provide an absolute limitation of the maximum amountof UL resources the WTRU may use. The second grant is the relative grantwhich directs the WTRU to increase or decrease the resource limitationcompared to the previously used value. Absolute grants are sent by theserving E-DCH cell. They are valid for one WTRU, for a group of WTRUs orfor all WTRUs in the cell. This is done by allocating up to twoidentities (called “primary” and “secondary”) for each WTRU, and by theUTRAN allocating the same identity to a group of WTRUs. Relative grantsmay be sent by the serving and non-serving node-Bs as a complement toabsolute grants. The WTRU behaviour is exactly the same for relativegrants for one WTRU, for a group of WTRUs and for all WTRUs. Therelative grant from the serving E-DCH RLS may take one of the threevalues: “UP”, “HOLD” or “DOWN”. The relative grant from the non-servingE-DCH RL may take one of the two values: “HOLD” or “DOWN”.

The following information is provided by the WTRU to the BS to assist inthe scheduling grant allocation. This information is provided in thescheduling information (SI). The logical channel ID of the highestpriority channel with data in buffer identifies unambiguously thehighest priority logical channel with available data and QoS informationrelated to this indicated logical channel. Some examples of informationin the SI include the WTRU Buffer occupancy (in bytes), total bufferstatus, buffer status for the highest priority logical channel with datain the buffer, as a fraction of the total reported buffer, and WTRUPower Headroom (UPH). The UPH field indicates the ratio of the maximumWTRU transmission power and the corresponding DPCCH code power.

HSUPA Serving Grant Functionality in Cooperative Networks

It should be noted that the objective of the Serving Grants is toprovide significant enhancements in terms of user experience (throughputand delay) and capacity. Therefore it is important to make sure that theserving grant functionality and objectives hold true when HSUPA is usedin a cooperative environment. Moreover, grants are a function not onlyof the required QoS but also the channel conditions.

It should be noted that the UPH is a function of the channel conditionsbetween the WTRU and the BS—if the conditions are not favorable, toomuch power is spent on the DPCCH and little power is left to the EDPCH.This is important because it implies that a grant between the WTRU andthe BS might not be necessarily appropriate for communication betweenthe WTRU and the relay.

Link Between BS and Relay

An important piece of the communication is the link between the relayand the BS. For example, if the bandwidth available for communicationbetween the relay-BS link is lower than the bandwidth available forcommunication between the relay and the WTRUs, the system might becomeunbalanced and the relay will start to queue and possibly drop the WTRUpackets because it is not able to forward such packets to the BS.

Some signaling messages between the relay and the BS are defined in thetable below.

Direction Message Description BS −> relay Measurement Request forchannel measurements, Request with specific reporting criteria (e.g.,periodic, event triggered). These measurements include UL DPCCH receivedpower for specific WTRUs, total power, interference, etc. relay −> BSmeasurement Response to “measurement report request” message Thesemeasurements include UL DPCCH received power for specific WTRUs, totalpower, interference, etc. BS −> relay relay polling Requesting status ofthe relay relay −> BS polling Response to “relay response polling”message. A response indicates that the relay is “In Service” BS −> relayload Request for number of WTRUs request associated to that relay, withspecific reporting criteria (e.g., periodic, event triggered) relay −>BS load Response to the “load request” response message This messagecontains the number of WTRUs associated with the relay, and possibly thebuffer occupancy of each of these WTRUs

The objective of these messages is to help the BS to perform allocationsto the WTRUs associated with a single relay, allocations forcommunication between each relay and the BS, and balance the allocationsbetween WTRUs associated with different relays.

Example 1 WTRU Communicates with Relay Only Centralized Scheduling

In the centralized scheduling the BS allocates a scheduling grant (SG)to each WTRU and this SG is sent from the BS to the WTRU via the relay.The relay will simply forward the allocation to the WTRUs.

Because the WTRU communicates directly with the relay, the SI sent bythe WTRU will reflect the link between the WTRU and the relay and the BScan use that information to perform the grant allocation. However, theBS also needs to take into account the fact that the relay needs to sendthe data from all associated WTRUs to the BS. Therefore, there is noadvantage in providing a large grant to the WTRU if there is not enoughbandwidth between the relay and the BS.

In order to account for that, we introduce a “relay SI,” which willreflect the capacity of that relay. The “relay SI,”, together with thehigher layer signaling messages defined above, can be used to controlnot only the SG allocation to the WTRUs but also the bandwidth allocatedto the BS-relay channel.

FIG. 51 is a diagram of an example frame structure for an SI 5100.Referring to FIG. 51, the frame structure for SI reporting from therelay to the BS includes a relay SI 5110, at least one WTRU ID 5120, andat least one SI 5130.

Hierarchical Scheduling

In the hierarchical scheduling the BS assigns grants to the relays,which in turn, based on the grant received, the SIs received from theWTRUs, and some other QoS information related to the WTRUs, assignsgrants to the different WTRUs associated to that relay. Note that inthis case control of serving grant allocation to the WTRUs is given tothe relays.

In order to guarantee that the BS assigns enough grants to the relays,the relay needs to send a “combined SI,” which contains the SI combinedfrom all WTRUs.

Difference Between “Combined SI” and “Relay SI”

Note that the “combined SI” and the “relay SI” described above may bedifferent because they serve different purposes.

The “relay SI” is used to reflect the capacity of the relay, so itcontains information such as how full the relay buffer and the channelcondition between the relay and BS. The BS then uses that information inconjunction with the WTRUs' SIs.

The “combined SI” contains information related to the buffer and channelcondition for all WTRUs combined, so that the BS can allocate enoughresources to the BS, which will then divide that among the WTRUs. Inthat case the BS uses the “combined SI” (and not the WTRUs SIs) toassign the Serving Grant to the relay.

Note that for the hierarchical scheduling, the relay could also send the“relay SI” information to the BS, in which case the BS could use boththe “combined SI” and the “relay SI” to perform grant allocation to therelay.

In other words, the “relay SI” contains information necessary to controlthe communication between the relay and the BS (used for both thecentralized and hierarchical case). The “combined SI” simply replacesthe WTRUs' SIs by combining information for all WTRUs in one SI (usedfor hierarchical case only).

Example 2 WTRU Communicates with Relay and BS at the Same Time

For the case where the WTRU is communicating with both relay and BS, thescheme might become more complicated. The WTRU may receive the grantdirectly from the BS or from both BS and relay. Using the same grant forboth links (WTRU-relay and WTRU-BS) might not be optimal since thechannel conditions are different. One option would be to use a combinedapproach where the WTRU receives grants for both relay and BS, and ituses the lesser received value. Another option would be for the WTRU tosend an SI that reflects the most conservative case (lower powerheadroom). However, this might not be optimal because it might limit thethroughput, since the grant defines the ETFC to be used (amount of datato be transmitted).

Centralized Scheduling

One proposed approach would be for the BS to assign the grant based oncombined information between WTRU and relay. Modified SI information maybe provided by the WTRU, which reflects UPH related to the channelbetween WTRU and the BS and between the WTRU and the relay. Theinformation may be provided by the relay with indication of the channelconditions, such as interference level and received power, between therelay and its associated WTRUs. Knowledge of the conditions of the linkbetween the relay and the BS may be provided.

Hierarchical Scheduling

Another approach would be for the BS to assign grants to the WTRUs forcommunication between WTRU and BS and to the relays, which in turnassigns grants to the WTRUs for communication between the WTRU andrelays. The WTRU would then have to handle grants from both BS and relayand somehow coordinate them. The method for coordination will depend onwhether or not the WTRU should use the same or different ETFCs for eachlink (WTRU-BS and WTRU-relay). If different ETFCs can be used, then theWTRU can apply the grants independently. Otherwise, the WTRU needs tomerge the grants, and the weakest link will dominate the transmission.This issue is discussed below.

Choosing Between Example 1 and Example 2

In the case where different ETFCs may be used to transmit data betweenthe WTRU and the BS and the WTRU and the relay, then different grantscan be applied to each link. In the case where the same ETFC needs to beused to transmit data from the WTRU to BS and from the WTRU to relay,then the same grant should be applied to both links. This will limit thethroughput performance. If the link between the WTRU and the relay ismuch better than the link between the WTRU and the BS, and if the grantwill be limited due to the poor communication between the WTRU and theBS, it might be better to choose to have the WTRU communicating onlythrough the relay, instead of through both relay and BS. In this casethe WTRU can take advantage of the good channel condition between theWTRU and relay and maximize its throughput. This is shown in Example 1discussed above.

LTE and Cooperative Networks

In LTE, channel allocations in the uplink are also done via usage ofgrants. Even though the specific details of the procedure for grantallocation are still evolving in the standards, it becomes clear thatthe issues and proposed approaches presented in this paper would also beimportant and applicable to LTE, with the appropriate modificationsrequired for channel allocations in LTE. Note also that, in LTE, theuplink transmissions are always sent in a shared channel (with the usageof grant allocations), in which case the issues described in this paperbecome even more important for LTE operation in a cooperative network.

FIG. 52 is a diagram of an example synchronization of the BS and RS DLtransmissions to the WTRU using timing adjust procedure 5200. Referringto FIG. 52, the BS signals to the RS 5210 and estimates the BS→RSpropagation delay 5220. The BS then signals the timing adjust value tothe RS 5530. The RS may then adjust the DL transmission timing 5240.

Although the features and elements of the present disclosure aredescribed in the embodiments in particular combinations, each feature orelement can be used alone without the other features and elements of theembodiments or in various combinations with or without other featuresand elements of the present disclosure. The methods or flow chartsprovided in the present disclosure may be implemented in a computerprogram, software, or firmware tangibly embodied in a computer-readablestorage medium for execution by a general purpose computer or aprocessor. Examples of computer-readable storage mediums include a readonly memory (ROM), a random access memory (RAM), a register, cachememory, semiconductor memory devices, magnetic media such as internalhard disks and removable disks, magneto-optical media, and optical mediasuch as CD-ROM disks, and digital versatile disks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, application specific integrated circuits (ASICs), fieldprogrammable gate arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module.

1. A method for wireless communications comprising: splitting a messageinto a first portion and a second portion; transmitting the firstportion of the message directly to a base station at a first frequency;and transmitting the second portion of the message to a base station byway of a relay station at a second frequency.
 2. A method for wirelesscommunications comprising: using a rateless 2-hop scheme at a wirelesstransmit/receive unit (WTRU); receiving a first plurality of informationbits from a base station during a first phase of the communication;decoding the received first plurality of information bits; receiving asecond plurality of information bits from a relay station during asecond phase of the communication, wherein the second plurality ofinformation bits are the remaining bits that the WTRU did not decode inthe first phase.
 3. The method of claim 2 further comprising: using adistributed antenna scheme, wherein the WTRU receives differentinformation bits from the base station and the relay stationsimultaneously; and the WTRU using successive interference cancellationto distinguish between the different information bits.
 4. A method forwireless communications comprising: receiving a message intended for awireless transmit/receive unit (WTRU); decoding the message; recodingthe message; and retransmitting the recoded message to the intendedWTRU.
 5. A wireless transmit/receive unit (WTRU) for use wirelesscooperative communications comprising: a processor configured to split amessage into a first portion and a second portion; a transmitterconfigured to transmit the first portion of the message to a basestation at a first frequency and transmit the second portion of themessage to a relay station at a second frequency.
 6. A wirelesstransmit/receive unit (WTRU) for wireless cooperative communicationscomprising: a receiver configured to receive a first plurality ofinformation bits from a base station during a first phase of thecommunication and a second plurality of information bits from a relaystation during a second phase of the communication; and a processorconfigured to decode the received first plurality of information bits;wherein the second plurality of information bits are the remaining bitsthat the WTRU did not decode in the first phase.
 7. The WTRU of claim 6further comprising: a distributed antenna scheme configured to receivedifferent information bits from the base station and the relay stationsimultaneously; and wherein the processor uses successive interferencecancellation to distinguish between the different information bits.
 8. Arelay station for use in wireless cooperative communications comprising:a receiver configured to receive a message intended for a wirelesstransmit/receive unit (WTRU); a processor configured to decode andrecode the message; and a transmitter configured to retransmit therecoded message to the intended WTRU.